Intrinsic Vow

Harry's hand halted.
He had a distant sense he was behaving uncharacteristically of himself, somehow. Like there was something he'd forgotten, something important, but he was having trouble remembering what it was, exactly.

—Narration, Harry Potter and the Methods of Rationality

A character has something he believes in more than anything. A person he has sworn to protect, an ideal he's sworn to uphold. He might have spoken that vow in a sacred ceremony, or simply made a decision in the silence of his heart, it doesn't matter. What matters it that no matter what, he will not break it.

If he's under More Than Mind Control, attempting to make him break it will cause him to break free of the control instead. If he's got Laser Guided or Easy Amnesia, he'll stop in his tracks without knowing why - or possibly even remember everything. If he's in there somewhere, this will enable him to fight.

Compelling Voice, Charm Person, Grand Theft Me, Brainwashed and Crazy, it doesn't matter - that one unbreakable vow will tear through anything. If the control exerted on him is particularly strong (or the show particularly cynical), he will only manage to retain enough control to commit suicide rather than break his vow - or his body might simply resist to the point where his mind or heart breaks from the stress.

And that, essentially, is the core of this trope - a character-trait so important to the character that he will break free of outside control or amnesia to keep it. Simply having a vow will not suffice, if it has not been 'tested' in this manner.

Unnervingly, the one way to get a character to break such a vow is to force them to choose between the lesser of two evils, with the greater being keeping the vow, making it a Frequently-Broken Unbreakable Vow. Subtrope to Heroic Willpower. See also Heroic Vow.

Examples of Intrinsic Vow include:

Anime & Manga

In Pandora Hearts, one episode sees Gil entrapped by a monster capable of controlling people with strings (like puppets, y'know). While it initially manages to make him smack around Oz, his best friend/master whom he's sworn to protect, Oz correctly reasons that Gil would never be able to kill him, and literally forces Gil's hand - which causes Gil to break free of the strings long enough to gun down the controller.
- Gilbert's devotion to his master Oz is so great that other characters twist it to their advantage.
Subverted in Code Geass - Euphemia hesitates and resists heroically when given the order to kill the Japanese, but the geass is too powerful for her willpower to break through it.

Comic Books

Not so much a "vow", but Wolverine was once snapped out of believing he was a 10 year old boy when a can of beer fell on his head. One could almost hear the "Popeye" power-up theme tune as he stares at the can and his features harden.
Superman was once hypnotized by Poison Ivy. Batman's only advantage was that Superman will NOT take a life... and that he will drop EVERYTHING to save Lois Lane, who Batman had Catwoman throw off a nearby roof.

Fan Fiction

Harry Potter and the Methods of Rationality: Harry is completely under the Dementors' thrall, and not himself. He moves to kill some people, and then...

He had a distant sense he was behaving uncharacteristically of himself, somehow. Like there was something he'd forgotten, something important, but he was having trouble remembering what it was, exactly.

In Winter War the Hollowfied shinigami show signs of this. Despite most of them being broken into Tortured Abominations, their instincts are still there. Examples include Kira refusing to hurt Hinamori, Hitsugaya turning against Ichimaru's control when he sees Hinamori's bleeding hand, and Renji calling off his attack when his Captain Byakuya orders him to stop.

Film

In Tron: Legacy, Tron stops attacking Sam as soon as he draws blood, because only users have blood, and he was originally programmed to defend them.
In Aeon Flux, random passersby sometimes completely lose their composure and break down in tears. It's the Power of Love overriding several hundred years of cloning.
- I thought that was just that they saw someone who looked just like a loved one did when they were that age.
  - Yes, but because the romance happened in a previous life, they're unable to remember why that supposedly random person in the crowd triggers such a strong emotional response.
In Terminator 3: Rise of the Machines, this exchange takes place when, under the influence of the T-X's reprogramming, the T-800 is holding John Connor down, ready to kill him.

John: What is your mission?
T-800: To ensure the survival of John Connor and Katherine Brewster.
John: You...are about to fail that mission.

Literature

In one of the later Artemis Fowl books, Butler is put under the Mesmer by an enemy. When she orders him to kill his friend/comrade-in-arms Holly Short, he resists enormously, and she eventually settles for having him render her helpless, rather than risk this. Later, she orders him to kill Artemis, whom he has protected with his life literally since birth. Butler promptly goes into cardiac arrest rather than obey. (Fortunately, Artemis is, like, RIGHT THERE, and of course he's got a plan.)
In the later Dune books, it's revealed that Gholas - who are basically resurrected corpses who possess the skills, but not memories, of their original life - can regain their memories if put in a situation like this. The original case involved Duncan Idaho, an extremely loyal follower of the Atreides, being ordered to assassinate Paul, the then-head of the House Atraides.
- Later, with more advanced technology, it becomes possible to clone Gholas from tissue-samples. Same rules apply, and it eventually becomes standard procedure to put the Gholas through simulated 'no-way' scenarios in order to force their latent memories to the surface.
The Dark Elf Trilogy reveals that Drizzt Do'Urden's father was, like himself, a rare 'Good' Drow Elf. After Drizzt turns his back on his people and escapes from Menzoberranzon, vengeful High Priestesses of Lolth brings his father back from the dead as a sort of semi-zombie - undead, but still possessing the skills of his former life. The reason being, of course, that Drizzt has proven too fearsome a fighter for all their assassins so far, so they figure that bringing back the man who taught him to fight might be their only shot. During the final confrontation between Drizzt and his zombified dad, pops managed to break free of the priestesses' control for just long enough to give a final farewell to his son, and then jump into a pool of acid so he wouldn't be able to hurt him.
Subverted in High Deryni, when Wencit of Torenth has physically and mentally tortured Derry, he tells Derry that he'll make him do anything he wants, then proceeds to demonstrate this by making Derry stab himself nearly to death. Wencit also assures Derry that he can make him betray his liege lord and friend Alaric Morgan. Wencit even leaves the dagger with Derry, asserting that his control is so complete he cannot kill himself unless he Wencit wills it. After Wencit leaves his cell Derry does try to kill himself to avoid betraying Morgan, but finds he cannot do so and weeps in despair.
Hollow Kingdom Trilogy: Kate tells her enchanted husband that she is pregnant, thereby stopping him from killing her, since the Heir is the most important thing to the Kingdom. Also, all the other enchanted goblins can't kill Kate because no goblin could ever think of harming the King's Wife.
In Randall Garrett's Lord Darcy stories, anyone who becomes a King's Messenger takes a vow (and is given a mind-control treatment to back it up) so that they not only cannot reveal a message they have been given in their official capacity, they don't even know what it is. Trying to force a King's Messenger to reveal a message in any way (via magic, drugs, torture, whatever) results in his immediate death.
Katsa in Graceling has an awesome moment of this. When the villain is using his hypnotizing voice on her, she is completely falling for it... until he threatens to reveal Po's secret, at which point she throws a knife through his throat. Best part? She didn't even remember what the secret was, just that it was Po's and the surrounding people could NOT know about it.
In Animorphs Mr. and Mrs. Chapman willingly became Yeerk hosts on the condition that their daughter be spared. When the Yeerks mention planning to infest their daughter anyway, the Chapmans immediately start fighting back against their control with greater strength than any character had ever displayed in the series. This confuses and frightens the Yeerks so much that they leave the girl alone just so it won't happen again.

Tabletop Games

In Nobilis, having a Virtue is basically this; someone with the "Vegetarian" virtue can't be tricked into eating meat, someone with "Egocentric" can't forget who they are, etc.
- In third edition, any belief you choose as an Affliction is this; it is an immutable rule of who you are.
This is built into the Charm Person and Dominate Person spells in Dungeons & Dragons.
There are many variations of Mind Control and one or two versions of Grand Theft Me in Deadlands. Under the classic rule set, virtually every one of them allowed a character a second chance to fight back if they are compelled to do something sufficiently "against their nature," which is left to the Marshal's judgement of the character. Effectively, the rules assume that almost everyone has one or two Intrinsic Vows.

Webcomics

In Order of the Stick, Haley breaks a paladin out of a suggestion spell by tricking him into thinking he had just attempted to kill the lord he had sworn to serve and protect.