Long-Lost Uncle Aesop

"And really, what did you expect them to do? Kill off Skippy? Or take time out from earlier episodes to throw in some random friend named Greg, just to give his death some meaning? Come on, it was the ‘80s. That time was much better spent on original songs by Tina Yothers."

—The Agony Booth, on the Family Ties episode "A, My Name is Alex"

The favorite relative/best friend character who appears in only a few episodes or just one Very Special Episode, was never mentioned before, and is never heard from again. They usually provide An Aesop, like "drunk driving is bad" or "beware of strange adults."

The purpose of these characters seems to be delivering the moral without having to inflict the issue on a regular basis, or, in the case of a fatal Aesop, kill off anyone important. You get the 22 minutes of angst, but the writers never have to deal with it again. Long Lost Uncle Aesops don't sit well with shows that have loyal and obsessive fandoms, and are incompatible with the Economy Cast.

Please keep in mind that this trope isn't just the sudden appearance and disappearance of characters who would logically be significant, but when they appear, drop their Aesop, then go back into the aether. See also Compressed Vice (when the issue is inflicted on a regular for one episode), Remember the New Guy?, Forgotten Fallen Friend, New Neighbours as the Plot Demands. Gay Aesops tend to be this as well.

Examples of Long-Lost Uncle Aesop include:

Anime and Manga

Sailor Moon did this a lot, since the characters often doubled as a Victim of the Week. Was especially jarring as it was occasionally suggested the leads have trouble making friends and seemed to have no real social life outside their small group, despite meeting and getting along with people fairly quickly each episode.
- One specific odd example is Makoto's friend Shinozaki. She says he's the closest person to her and their relationship is deeper than love—to the point that Makoto destroys the cardian that hurt him all on her own in rage—but he only shows up in one episode and the senshi, Makoto's other best friends, have never even met him. Then he disappears again.
A rather extreme example: The Dragon Ball Jump Anime Tour Special (later broadcast online) introduced Vegeta's outcast brother Tarble, who came to Earth looking for his brother's help. The special aired in 2008, twelve years after the original anime ended.
Kim in Fist of the North Star, who was Ryuken's fifth student before being expelled from his dojo, is only shown in a flashback to emphasize Kenshiro's compassion to others, and is never mentioned anywhere else in the series.
Himitsu no Akko-chan, the original 1969 series, introduces into the small crew of friends of the schoolgirl Akko-chan a deaf-mute kid. He'll never be seen after episode 32, and his only role is apparently stir Akko-chan's compassion enough to wish on herself deaf-muteness to better understand him, only to start breaking apart when, unable to reverse her own wish, she finds herself unable to handle the condition her new friend was bearing. Upon imparting the main character the much needed Aesop, the kid abruptly vanishes, and he's never seen or talked about. Ever.

Comic Books

Aside from her trademarked obsession with polka-dots, this was Little Dot's main gimmick: a never-ending assortment of uncles and aunts, most of whom had their own all-consuming passion e.g. Uncle Smoke and smoking.

Literature

A staple of the YA girls' series popular in the 1990s—not surprising, considering these books were sitcom episodes in print form. A typical example from the Babysitters Club series: Amelia, suddenly introduced as a close friend of Mary Anne, who had never been mentioned before and was killed by a drunk driver about ten pages in to teach the moral of the story.
Surprisingly averted with the character of Regina in Sweet Valley High. She dies as the result of a drug overdose, but up until then she had been a reoccurring character for several books.
The Apprenticeship of Duddy Krawitz, so much.

Live Action TV

Family Ties
- Alex P. Keaton's alcoholic Uncle Ned (played by Tom Hanks) actually makes three appearances, but two of those are a Very Special two-part episode. Originally the Trope Namer.
- Family Ties also addressed Teen Pregnancy, not by inflicting it on Mallory, but by bringing in Mallory's friend we've never seen before.
- Likewise, in another famed two-parter, Alex spends 40 minutes grieving over the sudden death of his close special friend... Whatshisname. Seriously, we'd never heard of him before, or ever would again. To a lesser extent, Season 3 featured brief appearances by "James" with the writers going to great lengths to try and convince the audience that he has been Alex's best friend/rival since childhood.
- Mallory would have a couple of these episodes as well. A "beloved" aunt who was like a best friend to Mallory ends up dying. Another episode had Mallory dealing with the mother of a friend who committed suicide. Neither the aunt or friend were mentioned before or after their respective episodes.
Matthew Perry's appearances as "Sandy" on Growing Pains.
Two and A Half Men has an episode about Charlie's best friend (played by Charlie Sheen's real life brother) dying from having a similar lifestyle to his. What's interesting is that such best friend had never been mentioned before and would never been mentioned since. In fact, Alan didn't even know him in spite of having lived at Charlie's house for quite some time by then.
On |M*A*S*H's first-season episode "Sometimes You Hear the Bullet", Hawkeye's happy-go-lucky best friend Tommy arrives just in time to get hit with the aforementioned noisy—and fatal—missile.
In the episode "Lie To Me" from season two of Buffy the Vampire Slayer, there is Ford, Buffy's "best friend" from her old high school in L.A. Ford is dying of a brain tumor, and bargains with Spike to be turned into a vampire in exchange for Buffy. Justified in that Buffy had moved a year ago, presumably leaving some her friends behind, and she'd spent the summer break back there, offscreen.
- Another Buffy example is in "Killed By Death," when Buffy exhibits a phobic reaction to being in the hospital, and we learn that it's because of her cousin Celia's death when they were kids. They were close, and Buffy was traumatized as a result of watching her die. While some of this trauma is legitimately resolved in the episode, the lack of mention of Celia before or after makes the whole thing seem to come out of nowhere. It is particularly jarring because Buffy spends a great deal of time visiting the hospital in Season Five, and shows no phobia beyond an appropriate reaction to the immediate circumstances. Nor does the First Evil ever manifest in Celia's form, which would seem an obvious way to get under Buffy's skin if the writers still remembered the character. Granted, the actress playing Celia - a long-deceased child character - could not concievably return, since the First Evil was only prominent five years later. It might've worked in the Season 3 episode "Ammends", but then the First was focusing more on Angel than Buffy.
- Buffy subverts this in the episode "The Puppet Show," however. When Emily (the dancer) is killed, Cordelia claims that she has lost "like, my best friend" - although Emily had never been mentioned on the show before. A subversion because Cordelia refers to Emily incorrectly as "Emma" during this speech and Xander calls her on never really having known the girl.
Scrubs takes place in a hospital, with a constant flow of patients, a setting almost tailor-made for introducing guest stars and Aesops. For some lessons, the emotional impact or context appropriateness of a random patient may not work for the Aesop so they'll say Remember the New Guy? and bring in a unmentioned family member or a hospital colleague who somehow never made it on camera.
- An interesting case is the patient trapped in an MRI machine with her face obscured. Cue the Aesop about taking chances for about the third time. Then they start dating and she appears in a later episode and they bring her into another plot and another Aesop by having her work as a social worker (or similar) who deals with patients. The two episodes could have easily been written for two different characters. And then she had a pill addiction, making it a threefer.
- Ryan Reynolds played J.D. and Turk's never heard of before college roommate. Justified in that this was only halfway through the second season, but he's never heard of again despite the fact that the rest of the series has dozens of flashbacks to their college days, and despite his character being notable for accidently telling Doctor Cox that Jordan's baby was his, which was a fairly significant plot point. Likewise, Doctor Cox in season 4 is given an old black best friend who Cox discovers has an autistic son, and promises to help out, but he is not seen again either (notably, Cox also passes him on to another old friend of his, who we don't see at all, who specialises in helping autistic kids).
- It happens with their families too. Carla's brother has the distinction of lasting most of a season, but afterwards is never heard from again, not even to meet his baby niece; her sisters only make cameo appearances in a couple of episodes. Turk's brother gets one episode, and phones Turk in another confirming that he can make Turk's wedding (though as far as the audience can see, he doesn't). Their families do get mentions, but they seem to miss nearly every important occasion. Averted with J.D.'s brother Dan, who is a semi-reccuring character, and Elliot's parents who are mentioned frequently and appear on occasion. Justified with J.D.'s father who was respecfully killed off thanks to Actor Existence Failure, and Doctor Cox's sister who he simply doesn't like to talk about as she reminds him of their painful, abusive childhood. Also justified with Jordan's sister Dani, who was a recurring supporting character but vanished from the last seasons- this was mostly in-character.
- The Janitor likes to plays with this trope. In one episode he puts on a fake moustache and denim jacket and tries to convince everyone that he is his own twin brother (nobody buys it); in another he pretends a nearby child is his son to make J.D. feel bad about insulting him. In a third he mentions his father died, causing J.D. to call him out by point out that he had met his father in a previous episode; the Janitor merely responds that "you met a man".
All Star Trek series have featured entire Aesop species who were brought in to visit their one problem and have the humans fix it or at least tell them what their problem was. One of the most overt examples was an early TNG with a race of drug addicts supplied by a race of providers. It falls short as an allegory of modern drug addiction as the addicted species were led to believe the drug was a treatment for a disease.
That '70s Show episode "Eric's Friend" featured a new gay character. Even though he and Eric became good friends during the episode, he was never seen or referenced again.
Mighty Morphin Power Rangers did this sort of thing a lot. One episode featured Kimberly's friend who's deaf (revealing that the Pink Ranger knows Sign Language in the process), appearing just in time to be immune to a music-related Monster of the Week's powers, while dropping an Aesop on being nice to people with disabilities. One could argue that the girl could be a friend from school, but she still came out of the blue.
Full House had Uncle Jesse's Greek Grandpa Papouli show up, have fun with everybody, then die about midway through the episode. They mourned, the Very Special Episode taught us how to cope with old people dying, and Papouli was NEVER MENTIONED AGAIN since. (I'm surprised no one here even remem...oh, screw it).
in Robin Hood, we learn (for the first time) in the fourth-to-last episode of the entire show that Robin's never-before-mentioned father and Guy's never-before-mentioned parents were all killed together in a fire. Except that Robin's father Malcolm didn't die, and he's actually been wandering Sherwood all this time without ever bothering to help or get in contact with his own sons or the two orphaned children of the women he loved. He reappears to tell Robin and Guy that they need to work together in order to rescue their half-brother Archer.
The George Lopez Show did this often with guest stars, especially with Carmen's friends and boyfriends, examples including Carmen's boyfriend of one episode with an Aesop about being pressured into sex and Carmen's best friend of one episode telling her about the trend of wearing different colored bracelets to symbolize different places you had sex.
Lucy's friend Sarah in the 7th Heaven episode "Nothing Endures but Change" had never been seen or even mentioned before the episode. (However, she did get a mention three seasons later, by Lucy herself while she acted as grief counselor to her friend Mike and his shell-shocked mother.)
In the Glee episode "Laryngitus", when Rachel loses her voice, Finn takes her to meet a quadriplegic friend of his who became paralyzed after an accident playing football. His Aesop was that he lost his ability to do what he loves most, but is working around that in order to live a fulfilling life. What makes this example particularly bad is that Glee is known for keeping track of its recurring characters, no matter how minor.
Nathan Kress played one of these—a wheelchair bound accident victim—on The Suite Life of Zack and Cody.
The Secret Life of the American Teenager had Ricky's never-mentioned former foster brother Ethan show up so the show could have a very special episode about sexting.
In one Very Special Episode of Boy Meets World, Shawn and Cory help protect a classmate who is being physically abused by her dad, and at the end of the episode she is sent to live with her aunt in another state. This girl was never seen before this episode and is never mentioned again.
A first season episode of Dawson's Creek featured a character named Mary-Beth who was supposedly a friend of Dawson's with a crush on him.

Western Animation

Apart from Static and Gear, most of the good Bang Babies on Static Shock simply stuck around long enough for us to learn their Special Problem, then disappeared into the ether.
- The show also had a rather Anvilicious episode about school shootings, where Virgil and Richie befriend a boy who's badly bullied by a popular Jerk Jock. However, we have never seen these two before this episode and never see them again afterward, despite the latter's alleged popularity and the lengths Virgil and Richie go to befriend the former. Considering that the series is about an accident that results in gang bangers (and a few bystanders) getting superpowers, this was to be expected.
Family Guy
- In"Peter, Peter, Caviar Eater," Lois's Aunt Margaritte comes to visit right at the beginning of the episode, but drops dead even before completing her first line, right there at the front door. On the commentary track, Seth MacFarlane explains that this scene was a direct parody of characters like this.
- Aunt Margaritte does show up in the time-travel episode, too, though.
- Family Guy has also played this trope straight on a few occasions. Lois' brother appeared in one episode as a maniac (also doubling as a Monster of the Week) before never being seen again. Lois' sister showed up in one episode to have a baby before never being seen again, though she at least gets occasional references in dialogue.
- She appeared as a main character in a recent episode... where an aesop was delivered and she disappeared again.
The Tonight Someone Dies episode of Clone High introduced - and killed - Ponce de Leon (portrayed as The Fonz and voiced by Luke Perry) as JFK's inseparable best friend, despite the fact that JFK was a main character who had already appeared extensively. Considering this is Clone High, however, this was the joke to begin with.
Jem did this a few times for Very Special Episodes. For example, Laura Holloway, who only existed in the episode "Alone Again" to get addicted to drugs and teach An Aesop.
Cousin Joss Whedon in Kim Possible. The Broken Aesop learned is that Kim isn't really a hero because she can beat super villains in her sleep, but Ron is the real hero because he's a This Loser Is You who sucks and would follow Kim anywhere despite his fear.
Lampshaded in the South Park episode "Red Man's Greed." An unfamiliar boy named Alex Glick appears in the crowd throughout the episode expressing his concern for the fate of the town. After he delivers the Aesop at the end Stan finally asks who he is, and it's revealed that he's some guy who got to do a guest voice.
In a Captain Planet and the Planeteers episode, Linka visits her cousin who gets her hooked on drugs, and turns into an addicted zombie. Said cousin dies of an overdose by the end of the episode, driving home the anti-drug Aesop.
- On the villains' side, we have Robin Plunder and Hoggish Greedly, Jr, who each appear for one episode.
- There's also Bambi Blight in the episode where the Planeteers learned not to judge a person based on their family.
C.L.I.D.E., a robot character introduced in the Tiny Toon Adventures episode "Elephant Issues" (specificially, the short "C.L.I.D.E. and Prejudice") whose main purpose is to make the episode address racism (the antagonist, by the way, is Montana Max).
Fievel's Aunt Sophie from Russia in An American Tail's Animated Series Fievel's American Tails appears for only one episode, her Aesop is mainly just convincing Papa not to be so strict on his son.
Cruella's mother Malevola appeared in one episode of One Hundred and One Dalmatians and announced during a family reunion she'd leave her fortune to the De Vil who gets Dearly Farm for her. Cruella won but, once she hear her mother announcing her intention to move there, undid it all because, no matter how much she loves money, it's not to the point of enduring her for it. Once Cruella got bold enough to tell it to her mother, Malevola got proud of her for finally proving to be a De Vil and made Cruella her heiress anyway.