Knight Templar Big Brother

You have a younger sibling. Either full-blooded, half-blooded, or just adoptive (or, in some odd cases, more than one of those at once). Especially a little sister. The kid is wronged somehow. What do you do?

Do you deal with the offender in a calm, civil manner?

Or failing that, perhaps you are a little upset, yet you keep control over your emotions, and keep yourself as restrained as you can?

Or do you go after the offender, the offender's friends, anyone connected to the offender, as well as anyone who even looks like they could be an offender sometime in the indeterminate future and decapitate them all without mercy?

If you are the latter, then congratulations! You are officially a Knight Templar Big Brother (or Knight Templar Big Sister).

This trope is what happens when a character takes the Big Brother Instinct to the extreme. They don't just protect the younger sibling, they absolutely destroy anyone who'd so much as even look at them wrong. Much like the Knight Templar and Knight Templar Parent, the Knight Templar Big Brother often genuinely believes that what they are doing is right and fully beneficial to the younger sibling, which often results in a clash of siblings later on within the show or work.

See also: Big Brother Instinct, Knight Templar, Knight Templar Parent, My Sister Is Off-Limits. If the characters aren't actual siblings, it probably is Yandere.

Anime and Manga
"Lelouch:"
 * Lelouch Lamperouge from Code Geass. After his younger sister Nunally was wounded in a terrorist attack (that also killed their mother), he started a war and took over the world, all for her sake.
 * In Season 1 Mao, a Stalker with a Crush to Lelouch's partner C.C, kidnapped poor Nunnally and strapped a bomb to her. Lelouch was already very pissed off at Mao, since he first  and then did the same to  ; needless to say, Mao did not meet a pleasing end.

"Reever: "Section Chief Peck, I would advise you not to look at Lenalee that way." Peck: "Why? Does she have a lover?" Reever: "She has a brother.""
 * Bleach has a few examples:
 * Ichigo adamantly believes the reason a big brother is born first is to protect all the little siblings that follow. He will force-feed this belief by the sword if he has to to any big brother he catches not protecting their own siblings, as both Orihime and Rukia's respective big brothers discover.
 * Once he manages to resolve his conflicting vows, Byakuya goes from being an Aloof Big Brother to a Knight Templar Big Brother where his little sister, Rukia, is concerned.
 * Toshiro Hitsugaya and Momo Hinamori have been raised together (the character book of Souls also confirms their relationship is "familial" and the old woman who raised them is regarded by both as their grandmother). He is incredibly protective of her despite being the younger of the two to the point where he attacked a perceived threat to her with such force two nearby vice-captains were caught up in the power he unleashed (one of which was actually her). In the end, their relationship is exploited by the Big Bad in one of the biggest Kick the Dog moments in the entire manga, the consequences of which is him making a vow to become strong enough to protect her from everything.
 * Henry Wong from Digimon Tamers went from being the most rational and calm guy in the group to being temperamental, hasty and reckless when his sister ends up in the Digital World.
 * Both Keiichi Maebara and later,, from Higurashi no Naku Koro ni fall prey to this syndrome when their non-blood-related "sister", little Satoko Houjou, is abused by her uncle. Winds up leading to two of the most tragic endings of the series.
 * It is heavily implied that her did the same regarding the aunt.
 * Caren, the Aloof Ally of Mermaid Melody Pichi Pichi Pitch, starts dangerously close to this, having been perfectly fine until her twin sister was kidnapped. However, while she blames and berates the good guys as well as the bad, she does do a lot of reluctant rescues and admits that she was wrong instead of going off the deep end.
 * Another Knight Templar Younger Brother is  from Cyborg 009, towards his older sister  . When.
 * Ran Fujimiya in Weiss Kreuz is downright obsessively protective of his Ill Girl little sister Aya. He became an assassin in large part to pay her medical bills (she's in a coma), and he is not in the least bit rational about threats to her safety.
 * In Yu Yu Hakusho, Hiei reacts this way to his younger half-sister twin sister Yukina being kidnapped and tortured.
 * In Naruto, Kankuro becomes one of these as a part of his Character Development, reacting violently to anyone who badmouths his little brother Gaara in front of him and going after Gaara alone after Deidara and Sasori of Akatsuki kidnap him and Kankuro's team mates are nearly all killed in an explosion. Given that Gaara used to be an Ax Crazy psychopath who regularly threatened to kill Kankuro, this is quite a drastic change. Sadly, it all goes horribly wrong when poor Kankuro gets his arse handed to him by Sasori, and Sakura barely manages to save him.
 * There's also the Raikage. When his response was to.
 * Possibly one of the most well-known examples is towards  The whole
 * It's somewhat inverted in One Piece as it's the younger brother, Monkey D. Luffy, that deploys this behavior.
 * However, it can be played straight with the older brother Ace. When he finds out that Blackbeard wants to target Luffy, he isn't pleased at all.
 * Mad Scientist Komui Lee of D Gray Man is very protective of his younger sister, Action Girl Lenalee. So much so that Link, wanting to go into a room which Allen had locked him out of, threatened him with, "you leave me no choice. I am forced to tell Supervisor Komui that you are in a sealed room with Lenalee Lee," to which Allen quickly apologized and let him in. There is also an anime-only Filler episode where Komui mistakenly thinks that Lenalee is going on a date with one of the Order's scientists and has to be convinced to simply stalk them all over town instead of outright killing the guy. He then repeatedly tries to attack and is comically restrained, until eventually he manages to summon a huge robot to aid him.
 * An excellent summation:
 * However, it can be played straight with the older brother Ace. When he finds out that Blackbeard wants to target Luffy, he isn't pleased at all.
 * Mad Scientist Komui Lee of D Gray Man is very protective of his younger sister, Action Girl Lenalee. So much so that Link, wanting to go into a room which Allen had locked him out of, threatened him with, "you leave me no choice. I am forced to tell Supervisor Komui that you are in a sealed room with Lenalee Lee," to which Allen quickly apologized and let him in. There is also an anime-only Filler episode where Komui mistakenly thinks that Lenalee is going on a date with one of the Order's scientists and has to be convinced to simply stalk them all over town instead of outright killing the guy. He then repeatedly tries to attack and is comically restrained, until eventually he manages to summon a huge robot to aid him.
 * An excellent summation:


 * Ryo Mashiba from Hajime no Ippo was orphaned when he was in high school, so he had to raise his younger sister Kumi on his own. Being a borderline Sociopathic Hero, he ends up as this towards her, and it's both played for drama and for hilarity.
 * Subverted with Seimei Aoyagi from Loveless. To him, it doesn't really matter HOW you treat his younger brother Ritsuka. Seimei believes that Ritsuka belongs to him and will rid of anyone getting in his way.
 * This trope is revealed to be 's main motivation in Vision of Escaflowne.
 * Kayo's brother in the Vampire Princess Miyu TV series has a Big Brother Instinct at first and then becomes this, trying to kill Miyu for getting close to his baby sister. But to be fair,
 * In the manga of Death Note Villain Protagonist Light Yagami claims in an early Motive Rant that part of the reason he's killing criminals en masse is to make the world a safer place for his little sister Sayu.
 * In Hell Teacher Nube, the demon Bakki can bounce between Laughably Evil and evil... but if you hurt his little sister Minki, you. are. DEAD.
 * Deconstructed in Overman King Gainer: Asuham Boone will react violently (and disproportionately) towards any slight to his younger sister Karin, even minor or imagined ones. The whole reason why he wants to capture Gain and thwart the Exodus is that he had convinced himself that Gain had seduced and abandoned her, . It gets so bad that . He gets better, though... Perhaps too much.
 * When Last Order is kidnapped in A Certain Magical Index,  reverts back into the person he used to be, except in an effort to save her. Slaughter ensues. The lesson in all this? Don't mess with Last Order if you value your life.
 * Misaka has shades of this herself when she discovers what's happening to her sisters/clones and what she's willing to go through to stop the Level 6 Shift project.
 * from Tantei Gakuen Q become this, trying to protect  from a Smug Snake aunt who wants to become her tutor and steal the fortune she inherited from their Missing Mom alongside a Phony Psychic accomplice.
 * Both inverted and played straight in Fullmetal Alchemist. Edward is multiple times in the series confronted with the question of how far he's willing to go to save Alphonse, but sanity and morality usually win out. However, in an early episode of the 2003 anime adaptation, Alphonse comments that if something were to happen to Ed,
 * Speaking of Scar, he counted as one in the manga and second anime himself (In the first anime they had a rockier relationship). The realization of his brother's death sent him into an uncontrollable rage, during which time he . He later moves on to hunting down and murdering State Alchemists to avenge him.
 * Phoenix Ikki of Saint Seiya embodies this trope when his little brother Andromeda Shun is involved, to the point that putting Shun in life-threatening danger became the default way to work Ikki into any given storyline.
 * In Chrono Crusade, when Rosette Christopher's little brother is kidnapped by the Big Bad, she makes a Deal with the Devil and spends four years training as a Church Militant. The moment she discovers the location of her brother, she travels from New York to San Francisco, guns blazing, killing any demon that attempts to get in her way, all while being the only Badass Normal in the manga . In the manga, it all leads to a battle between her and who is strong enough to tear apart a massive army of demons with his bare hands. And she kicks his ass.
 * There's also Fiore, who considers herself as something of a big sister to Joshua. She seems gentle and demure at first, but when the time comes she doesn't hesitate to.
 * Durarara!!'s Shizuo Heiwajima tends to act like this in regards to his little brother, Kasuka. While he has reason to be concerned for his brother's safety and privacy (Kasuka's a popular actor, and has been stalked by both rabid fans and the paparazzi in the past), Shizuo takes it a bit too far by immediately becoming suspicious of any mention of his brother by strangers, innocent or otherwise. Said suspicion usually escalates into tree-throwing.
 * Touya from Cardcaptor Sakura has some shades of this because he promised his mother on her deathbed that he'd protect Sakura.
 * Inverted in Rurouni Kenshin in that Enishi is a Knight Templar Little Brother. So concerned about his older sister's well-being and so seething with pure hatred toward his brother-in-law for  that Enishi collaborated with a league of Shogun assassins in order to kill Kenshin - and this was while he was ten or twelve.   Enishi became a little psychopath before getting involved in organized crime in order to one day take Revenge on the man who.
 * Lets not forget Urd from Ah! My Goddess! Urd makes it clear that she will do anything and use any power she has to in order to protect her sisters, Belldandy and Skuld. Urd has to continually "punish" Aoshima after his latest rape attempt against Belldandy or Skuld (he tried to force himself on her when Skuld grew up after Yggdrasil crashed). Urd stood up to her own mother Hild, the Daimakaicho, telling her to stop tormenting "her little sister" Skuld after Skuld incurred her wrath. Since Belldandy goes absolutely postal if anyone even blinks wrong at Keiichi, it is easy to overlook that Urd is a Templar Knight Big Sister as well.
 * Akira's older brothers in Ai Ore Love Me are quite protective of him.
 * More than one Sympathetic Murderer in Detective Conan is a brother or a sister seeking revenge for the death or ruin of their younger or older sibling.
 * from Mawaru Penguindrum, as of episode 21.

Comic Books

 * Katina Choovanski is quite capable of taking care of herself, but her crimelord sister still keeps interfering in her life - sometimes by killing people horribly because they tried to hurt Katchoo, sometimes by trying to ensure that she ends up with the "right" love interest. Our heroine does not appreciate this.
 * Raphael, while not being the most even-tempered character, is psychotically protective of little brother Michelangelo - to the point where he states that the 'thought of Mikey being hurt makes him experience violent rage' (see The Other Wiki). And considering how violent Raphael is usually...
 * She Hulk and the Incredible Hulk are cousins, but they have a Brother/Sister relationship. She-Hulk can take care of herself most of the time, but pity the poor fool who harms She Hulk within proximity of The Hulk. She-Hulk herself can be extremely overprotective of The Hulk as well. Iron Man got his ass handed to him by Shulkie when she found out Stark exiled Hulk into space.
 * How about that time Nightwing freakin' killed The Joker with his bare hands when he thought he killed Robin? Of course, ol' Bats quickly revived him afterwards, but even still.
 * Well...
 * Quicksilver for the Scarlet Witch, except that he is her twin, not big brother. But recent portraying of Wanda's power suggest that if you attack Wanda, you are lucky if Quicksilver kicks your ass before she has to use her power.
 * Thor's brother Loki is a manipulative, deceitful god of lies and mischief, and he often does bad things and sometimes gets punished for them… but if you dare hurt him unjustly Thor will smite you from the face of the Earth.

Fan Works

 * The Kage Tenshi's The Red Monarch's Sousuke Miyabe is one for his little sibling Tatsuki. To put things into perspective, 10 years ago, he  Tatsuki saw all of this, which comes back in literal nightmares. To get the full story, if you read the spoilers, read The Red Monarch until you reach the most recent chapter, as it is still ongoing. The link directly to the story is provided for convenience.
 * In Murderer's Row, a Prison AU of Red vs. Blue, Grif and Simmons were thrown in prison for murdering a man who hospitalized Sister.
 * In Magica Madoka Veneficus Puella, Sayaka adopts an orphan. Someone kidnaps her. Do the math.

Film

 * The Blind Side: Michael is usually a Gentle Giant, but
 * The Godfather: Michael Corleone. Of course, that is the job description of a romanticized mob Chieftain. But if you ever go against the family again...
 * Of course this was before Michael decided.
 * When the protagonist of Sick Girl finds out her little brother's being bullied, she hunts the three bullies down, kills one of them, and forces the lead bully to drown the other, then ties him up in her barn and spends several days torturing him to death.
 * Dead Mans Shoes. Somewhat after the fact, older brother Richard returns from the army after discovering his brother has been bullied and goes about giving the bullies a taste of their own medicine with interest.
 * Haru from Beverly Hills Ninja: "I may not be the Great White Ninja, I may not be one with the universe but I know this...NO ONE MESSES WITH MY BROTHER!"
 * The titular character of Ginger Snaps starts out as a Knight Templar Big Sister and gets steadily worse over the course of the movie, as her transformation into a werewolf makes her more and more aggressive. For example, she kills the janitor because she thought she saw him look down Brigitte's top earlier in the movie.
 * J.J. Hunsecker in the dark Film Noir Sweet Smell of Success ruins his little sister's boyfriend's life because he doesn't approve of him for her.
 * In the 1938 Yiddish film Mamele, Khavkhe (Molly Picon) takes care of her whole family. A gangster, Max Katz, recruits her brother into a burglary gang, and courts her sister (with the intention of turning her into a prostitute). Khavkhe manages to pickpocket Katz's gun, and makes him write a letter breaking up with the sister. ("You may be a Katz (cat), but you'll leave here as a kitten.")
 * Four Brothers: Bobby Mercer is already pretty violent and short-tempered to begin with, but hurt his younger brothers and he WILL put you in either the hospital or a body bag. He proves this multiple times throughout the movie, especially when he goes on a murderous rampage against Detroit crimelord Victor Sweet and his cronies after.

Literature

 * In the Cal Leandros series, this is pretty much the whole premise of Niko Leandros, who would burn the world to protect his brother. The only reason Cal ever gets near to any combat is because Nico is a firm believer that the only way to be ready for combat is to actually participate in it.
 * from The Picture of Dorian Gray. His little sister  killed herself when he was working abroad, so when he found out, he swore to kill the culprit, Dorian Gray.
 * In Hells Children, by Andrew Boland, you never, ever, F%*& with Acheri, or her defacto big Brother, Precision, will Mind Rape you.
 * Gretchen in 1632.
 * Max in Maximum Ride. She's normally a calm and easy-going person. However, she treats the flock like her own brothers and sisters. If you're foolish enough to threaten one of them, you're gonna regret it.
 * Rish in Captain Vorpatril's Alliance is so much of a Knight Templer Big Sister to Tej that Barrayaran intelligence erroneously reports her as a genetic-slave bodyguard. She is willing to do anything to protect her including(being Jacksonian)some morally questionable things(which would probably include murder or torture should it be thought needed).

Live Action TV
""Whoever said Dean was the dysfunctional one has never seen you with a sharp object in your hands.""
 * Scully's big brother Bill. It's really annoying, since this is a woman who has been shown to be able to handle herself time and time again amongst a predominantly male-oriented workplace. She goes out of her way to never be accused of special treatment because she's a woman. And yet she lets Bill push Mulder around all the time.
 * It's not all the time. Bill and Mulder meet about twice in the entire course of the series, and though Bill bad-mouths Mulder, the fights between the men usually happen when she's not around. Not to mention Mulder is capable of holding his own in those fights. The annoyance of this trope comes more from the fact that Bill is making assumptions with half the story.
 * Also slightly justified when it comes out that Bill has always acted like this as a result of having to be "the man of the house" when his father was away at sea, and that Dana has always taken his opinion with a grain of salt. It's not only Mulder Bill has a problem with, but her having a life different from what was expected of her: joining the FBI rather than practicing medicine, not getting married, having a child out of wedlock...Mulder is only a part of it, though a substantial part.
 * And though his little sister is gone for the entire series, Mulder could most definitely count. He's spent his entire adult life searching for and avenging his sister's kidnapping. Samantha Mulder is the reason Mulder works on the X-Files and why he is so determined to uncover the conspiracy. He does not hesitate to hurt or kill someone in the process of finding her.
 * Big Bad Kanzaki Shirou from Kamen Rider Ryuki very definitely fits this trope.
 * Supernatural has the Winchester brothers. Both of them.
 * If you think of even putting a single finger on Sammy, well then you must be fist and gun proof. Or if you're an angel, messing with Sam to get Dean's agreement is a bad idea.
 * After making Sam watch Dean die over a hundred times in one episode to desensitize Sam to losing Dean (it doesn't work; after Lilith kills Dean, Sam turns to The Dark Side for vengeance and the power to save the world) the trickster comments that:


 * You probably don't want to f*** with Cas on Dean's watch (or vice-versa)either, although they are more like unofficially adopted siblings
 * Lucifer and Michael might have been trying to destroy one another, but Luci got pretty vengeful when Castiel molotov'ed Michael.
 * Firefly: Just don't mess with Simon's little sister.
 * For that matter just don't mess with River's big brother. Pity the poor Reavers.
 * No matter how he tries to negate it, John Casey seems to take that road in regards to Chuck, even though he always claims it's to protect the Intersect. The Casey Doth Protest Too Much, indeed.
 * Of course Casey's just protecting the Intersect. That's why . Which happened in the brief interim when Chuck wasn't the Intersect.
 * It is revealed in one episode of Malcolm in the Middle that Reese is this to Malcolm, and that was the only thing keeping him from getting beat up for his smart mouth. Malcolm finds this out when his brother decides to be nice for the rest of his life.
 * Don Eppes of Numb3rs. How many people were expecting him to kill someone during Breaking Point?
 * Justin, from Wizards of Waverly Place, tells his younger sister, Alex, that he always thought he would intimidate her boyfriends if they got out of line and in Wizards vs. Werewolves he fights Mason (who is in full werewolf mode) for her, with his bare hands.
 * Don't get in between Iris Crowe's little brother and... anything. Just don't.
 * On Burn Notice, do not pick on Nate. It is unhealthy.
 * In the episode "Comrades" there is a deconstruction. Michael has to convince his client that being a Knight Templar Big Sister might actually not be in the little sister's interest.
 * Actually Michael is perfectly fine with Knight Templar Big Brotherhood in that episode as in others and he puts the little sister's kidnapper through a more hellish experience then his client would have ever been able to think of. He just objected to being an inefficient Knight Templar Big Brother.
 * Prison Break: Don't mess with Linc or Michael will come up with an elaborate plan to destroy you. Don't mess with Michael or Linc will kick your ass.
 * Mycroft Holmes worries about his little brother Sherlock constantly. Since Mycroft is the British government, he gets to do his worrying by taking control of CCTV cameras and having his brother's friends abducted in conspicuous black vehicles.
 * Buffy the Vampire Slayer warns her friends in the season 5 finale that she will kill anyone who tries to lay a hand on her little sister, Dawn, True Companions included, even though Dawn's death might prevent the apocalypse, and Dawn is a magical creation only a few months old instead of a natural-born sibling.
 * Jing Tian from Chinese Paladin 3 boomerangs from having a simple Big Brother Instinct to full on Knight Templarhood. For example, his reaction on seeing his best friend "threatening" his little sister with a Healing Shiv is to attack and nearly kill him.
 * Cesare Borgia is this to a somewhat frightening degree in regards to his younger sister, Lucrezia. (Of course, a lot of fans would argue that there's some subtext to that protectiveness.)By the third episode he's threatening to cut her husband-to-be's heart out with a dinner knife if he's "ungallant". He shows some signs of this towards one of his younger brothers, Gioffre. Not so much with Juan.

Myths and Religion
"And thereupon he rushed under the horses and cut off their lips at the teeth, and their ears close to their heads, and their tails close to their backs, and wherever he could clutch their eyelids, he cut them to the very bone, and he disfigured the horses and rendered them useless. And this is just his initial reaction."
 * Absalom from The Bible, who learns that his sister Tamar was raped and humiliated by their half-brother Amnon and, instead of telling their father David to demand punishment, he goes and kills Amnon himself.
 * In all fairness, David was already well aware of what had occurred (see 2 Samuel: 22-23). His response was to be 'very angry'... and do absolutely nothing. Absalom decided to take justice into his own hands - it was obvious none was forthcoming from their father, the king.
 * In a similar incident two of the sons of Jacob killed everybody in a village for mistreating their sister. Presumably they wished the feud to end right away.
 * To put it into perspective, Jacob's daughter Dinah had been raped by a prominent member of the village who fell in love with her. Despite the circumstances, he and his father wanted to smooth over relations with Jacob's family and offered to pay for her dowry as if she'd still been a virgin, which was actually quite a generous offer at the time. Jacob seemed to warm up to the offer, but his sons demanded that all the men of the village circumcise themselves to become part of Jacob's tribe. The men agree, with the idea that they will now be able to intermarry with Jacob's tribe. After all the men are circumcised and helpless, though, Dinah's brothers take advantage of the situation and slaughtered the entire village, taking only women and children alive as slaves. When Jacob bemoaned them for the actions, saying that no one would trust him now, they only reply: "Our sister should not be treated like a whore." Damn!
 * The "extra" killings could be interpreted as Pragmatic Villainy just as easily as anger. If there is no one left there would be no one to take revenge.
 * In The Mabinogion, Efnisien gets very angry when his half-sister, Branwen, is married to the king of Ireland:


 * And when Branwen's husband starts treating her like a slave, all her brothers go to war to defend her honour.

Toys

 * The filecards for G.I. Joe members Barrel Roll and Bombstrike, and the filecard for Cobra operative Blackout contain a story where Barrel Roll acts as Knight Templar to rescue his sister Bombstrike after blacksheep brother Blackout kidnaps her.

Video Games

 * Gulcasa of Yggdra Union. Serious harm befalling his younger half-sister Emilia induces Berserk Button-level rage, and towards the end of the game—directly after he learns —said rage causes him to reject Yggdra's repeated pleas for a cease-fire to the war that has . Usually, he's a shrewd and reasonable ruler, but in this case...
 * Emilia actually inverts the trope in one of the routes of Blaze Union where Gulcasa never discovers his powers (or that they're related), and purposely risks going permanently Ax Crazy in order to get the power to protect him and the rest of her adoptive older siblings in Gram Blaze. On the chance that she actually does go insane,.
 * Don't get anywhere near The Legend of Zelda the Wind Waker 's Aryll, or prepared to be mercilessly hacked into pieces by the Master Sword. Good thing Link's an only child in all of the other The Legend of Zelda games...
 * Well, he may be an only child in all of the other Zelda games, but in The Legend of Zelda Twilight Princess, he practically treats the Ordon Village children as his younger siblings (Ilia may also be under this trope if considered "like a sister," but there are issues which would complicate such an application). In particular, when King Bulblin tries to recapture Colin in Kakariko Village, Link rides in visibly pissed on a level never before seen in the series. Things don't end well for King Bulblin and the other Bulblins. And in the long run, things really don't end well for.
 * Which is the main reason why Ilia can be considered an Expy of Aryll in terms of role. In terms of personality and looks, she's more of a cross between Malon and Saria though. Anyway, Link's loved ones always act as his Berserk Button, even if they don't happen to (yet) include Zelda.
 * In the non-canon Super Smash Bros.. Brawl story mode "The Subspace Emissary," Link witnesses Mario and Pit apparently kill and dissolve Zelda, and he breaks the sound barrier to fuck them up. If he's running that fast WITHOUT the Pegasus Boots, you are definitely in trouble.
 * Zelos Wilder in Tales of Symphonia. Though Seles seems to hate him for being born, he will "throw anyone who so much as looks at her wrong down the Palmacosta harbor". Whoa, dude, chill.
 * Don't forget Guy Cecil from Tales of the Abyss—his older sister Mary, but he practically raised Luke. Guy's a pretty chill guy normally, but if you so much as threaten one hair on Luke's head, you press his Berserk Button. Especially evident during the Summit, when   Okay, Guy, it was just a suggestion. Calm down. Easy, now. Just take a deep breath, it'll all be okay . ..
 * Guy had a pretty good reason for getting upset. Luke was massively depressed at the time, and was seriously considering the above suggestion.
 * from The World Ends With You makes a Face Heel Turn in reaction to
 * Targeting Axel's little brother is not a good idea, something a repo man learned the hard way at the end of the Axel story. And as aforementioned Dark Hero learned the hard way, do not abduct Adell's siblings or Rozalin's "servants". For any reason at all.
 * And let us not forget that Mega Man and Proto Man are basically teaming up together in 10 to get the vaccines for Roboenza, to cure Roll. Hell Mega Man goes after Wily before he goes after the Robot Masters. Whoever created this virus better have good life insurance.
 * Blues in Mega Man Megamix fits the 'morally questionable actions' part of the trope (what separates it from Big Brother Instinct), up to and including threatening to kill Kalinka Cossack if anyone gets involved in . Of course, he did succeed in saving everyone  There are plenty of other examples.
 * Consider that Wily based his own robot designs on Proto Man's specs. Bass would then be multiple inversions of the trope: he's on a constant mission to destroy Mega Man, and if Wily pisses him off he'll gladly take it out on the other Wily bots...who are also Proto Man ripoffs.
 * Any attempt to harm Mega Man X or Zero in the other's presence will result in the survivor of the pair bringing the Big Bad's fortress down on top of you.
 * In later games, if you take out Axl, you now have to deal with TWO pissed-off Knight Templar Big Brothers.
 * Prototype: Whatever happens, whoever you are, whatever the reason, never never never harm Dana Mercer in any way, because the consequences for doing so come in the form of her big brother, Alex. Alex is a shapeshifting, superstrong, superfast, superagile, incredibly resilient Humanoid Abomination with issues, and Dana is the one thing in the world he cares about. Alex once prevented Dana's capture by punching through the torso of a soldier wearing body armour. Alex ate a few dozen people who happened to be vaguely connected to Dana's location when kidnapped, and two monsters, and the mastermind herself. Alex is already not wholly stable. Try something safer, like playing Chicken with the sun.
 * In Persona 4, acting this way will get you the worst ending
 * Axel of Kingdom Hearts, towards Roxas. He raises the amnesiac keyblade-wielding Nobody... but he's also a really Poisonous Friend.
 * In Grand Theft Auto: San Andreas, CJ finds out that some construction workers have been calling his sister Kendl a hooker. He traps the foreman in a porta-potty and uses a cement mixer to bury him alive. And all the while, the poor guy is screaming "OH GOD, WHY?!"
 * Dirge of Cerberus messes with this. It's actually the little brother, Nero, who goes completely batshit when his big brother is threatened.
 * in Castlevania: Order of Ecclesia.
 * The protagonist of the Japan-only Nie R Replicant is The Unfettered when it comes to protecting his sister Yonah (in the Western version, he's instead a Papa Wolf).
 * In Dragon Age II, the player character can act like this towards a younger sibling, Bethany or Carver. And considering what can potentially happen to them throughout the game, it's no wonder.
 * A possible sidequest in The Godfather 2 involves you being told to beat up or kill someone taking advantage of the quest-giver's sibling.
 * A possible sidequest in The Godfather 2 involves you being told to beat up or kill someone taking advantage of the quest-giver's sibling.

Web Comics
"Jason: If you do ANYTHING to her, in any way, shape or form, I will quite literally #%"Jason: If you do ANYTHING to her, in any way, shape or form, I will quite literally #%{{quote|Jason: If you do ANYTHING to her, in any way, shape or form, I will quite literally #%$&!$# EAT YOU. And you have NO idea just how DEEP I could BURY what'd be left of your sorry carcass. UNDERSTOOD?}}!$# EAT YOU. And you have NO idea just how DEEP I could BURY what'd be left of your sorry carcass. UNDERSTOOD?" !$# EAT YOU. And you have NO idea just how DEEP I could BURY what'd be left of your sorry carcass. UNDERSTOOD?"
 * Johan of Anders Loves Maria has long been violently protective of his little sister, Maria. Leads to him committing acts of vandalism and assault in her name.
 * Mijuu of Juathuur is Dejoru's Knight Templar Big Sister.
 * Panthera has Jason, who threatens his best friend Fletcher (who finally got a date with Taylor) as such:

Web Original

 * Broken Saints: Tui Jr, adoptive brother to Heartwarming Orphan Shandala, can resemble this at times, due to his being so hot-blooded.
 * Lenny Priestly of Survival of the Fittest v3 will do anything to make sure his twin sister Elizabeth is safe. Literally anything, as he kills people for this reason. Though possibly a subversion, as it has been implied that he used her as an excuse to kill, especially since he went Ax Crazy and stopped being an Anti-Villain.

Western Animation

 * Kenny displays this in the episode "The Poor Kid". If you have the gall to mess with Karen McCormick, Kenny proves that he Would Hit a Girl quite viciously. The rest of the school gets the hint that the girl is off limits.
 * Dee Dee may appear to be a flake, but you do NOT harm her little brother. Or she will kick your ass.
 * While they're on separate sides, Proto Man certainly fits. No one, save Protoman himself, is allowed to touch Mega Man—not even Wily. True, he states that wants to defeat Mega Man himself, but not really.