Forgettable Character

Mainly a character that the other characters forget of his existence, mainly because the forgotten is probably so unremarkable, ordinary, and bland that he blends right into the background, basically a Ridiculously Average Guy, a minor villain, a villain that didn't leave some sort of impression on the heroes, or a mook that was easily defeated by the heroes early in the story, yet returns just to find that everyone forgotten who they are. Also, an unnoticeable character that has a low-key occupation or role that the cast can easily forget about him. Sometimes it takes a drastic turn for a character to be forgotten, like having some sort of power that makes everyone forget about them or that the villain casts a spell, uses technological brainwashing huge laser, just holds them hostage long enough that causes the whole universe to forget about this particular character. In political thrillers, spy novels, and any serious cases, the character wants to be forgotten to not imperil the others, so his or her foes would not harm them in any way shape or form, so he leaves his family and friends.

To other characters that might want to remember this person, may leave a self note just to help them remember.

Please note that this trope is mainly for in-universe only. The meta examples go to Out of Focus.

Anime and Manga

 * Canada of the Axis Powers Hetalia series, his interactions with other characters usually involved them forgetting who he is, his existence, or worst, mistakening him for North America.
 * Jewelpet Sunshine has three examples, Nejikawa who is forgotten by his classmates at least once. Yaginuma which is a debatable case, because of his limited screentime. And Katori, whose peers keep on forgetting that she's there, because she's tiny.
 * Tetsuo Ishimaru of Eyeshield 21, his teammates keep on forgetting that he exists. Tetsuo breaks the fourth wall just to comment on this.
 * In Saki, this trope is nearly a superpower to Momoko Touyoko. She's invisible to anyone in the vicinty, since she barely leaves an impression. There's an incident when she was playing Mahjong and the other three players forgotten that she's there despite sitting next to her.
 * In Sayonara Zetsubo Sensei, despite being the Class Representative, Kagero Usui has a property to just slip away from the other characters' memories. Just because his Prematurely Bald head is his only distinctive feature. - He takes great pains to hide most of the time.

Film

 * In Office Space, no one notices Milton, the quiet office drone, for several years before he was actually fired from the company. Worst is that everybody forgotten to tell him the news. Also, Milton continues to work there with no employment file, because the payroll system glitch and continues to send him a monthly salary (No one noticed this as well).
 * Randy from Liar Liar, whom Fletcher Reede doesn't even bother remembering.

Literature

 * In Harry Potter, Peter Pettigrew (aka Wormtail) was known as the quiet yet unassuming guy in the Marauders. Also the fact that he was killed supposedly by Sirius, yet nothing besides that. This trait is what made him trusted by Sirius with the Potter family secrets and
 * In Blindsight, the Captain is this trope. It's so bad that the protagonist treats this as a huge reveal that the Captain is in charge (It's no secret that the Captain is an AI, plus the protagonist is a walking Sherlock Scanner, who forgets the Captain's existance since it doesn't leave any clues).
 * In Vlad Taltos, Kragar is just utterly forgettable that no one will notice him, even though he's standing in the same room, speaking to them, and hasn't moved.

Live-Action Television

 * In Buffy the Vampire Slayer's sixth season, the Terrible Trio which consists of Jonathan, Warren, and Andrew, a new character. The creators wanted to add Tucker as the third member, but his actor can't be contacted. This played a part in making the character of Andrew as Tucker's younger brother who has similar powers, and was gone when he summoned flying monkeys to attack the school. By Buffy standards, this incident is totally mundane that none of the cast remembers it or Andrew, now it's a gag to refer to him as "The other guy" or "Tucker's brother."
 * In Doctor Who has the alien race known as the Silence, who have the power to invoke this trope. The minute a person or different alien turns away from one of them, their existence is forgotten. They're instantly remembered the next time when the same person lays eyes on them. This is seen as the Silence using this for infiltration, though their original purpose was
 * In Arrested Development, not one of the Bluth family (except for George-Michal, the sometimes boyfriend) remembers Ann Veal's name; they usually refer to her as "Bland," "Egg," "Annabell," "Yam," or "Her?" It's not done out of malice, it's just that Ann Veal is so forgettable.

Theater

 * In Of Thee I Sing's Alexander Throttlebottom, the Vice Presidential nominee, who is running with John P. Wintergreen. Alex is so forgettable that he has to sneak during the White House tour group, since no one remembers him. (Back then when the play was written in 1930s, the VP position was infamous for its lack of notability.) Though he's remembered in the grand finale, for Pair the Spares reasons.

Video Games

 * The Princess from Sakura Samurai, her royal subjects forgotten about her (Except for the friendly kappa that the player talks to). Justified since she was kidnap.
 * In some of the Mario games, Mario's younger twin, Luigi, suffers from this occasionally. Like in the Super Mario 64 DS remake, the Toads honestly admitted in front of him that he's not as memorable as Mario.
 * The Ace Attorney games has Winston Payne. In the past, he was fear as a rookie murderer, now Payne is unremarkable that most of the characters forgotten just who he is. Even when Detective Gumshoe just mentions him to Miles Edgeworth, one of Payne's fellow prosecutors (Who Payne insists that he mentored when Miles was younger), now Edgeworth assumes that Winston is just one of the janitors.
 * In Fire Emblem Awakening, there's Kellam. Lampshaded in his support conversations constantly and showing up in the gameplay: one would've never guessed that he'd be reclass into an Assassin.

Western Animation

 * In The Simpsons, there's many examples, like:
 * The Running Gag which has Mr. Burns forgetting who Homer is, though this was dropped mostly in later seasons.
 * Homer just as often forgetting about Maggie. Sometimes referring her just as "the baby", other times forgetting about her completely.
 * In Futurama, the running gag is that Scruffy has to reintroducing himself to people. Just to the point that "I'm Scruffy, the Janitor" also became his Catch Phrase. In addition, the other characters are just as forgettable to him, since he doesn't remember them either.
 * In South Park, "The Last of the Meheecans" episode, where the boys play a Border Control rpg, and the Mexican side won. It takes a long time, until Cartman realizes they forgotten about Butters (and even then only because he can win as the Texan side). The others wonder how this happened, Craig reminds them that Butters is easily forgettable.

Web Comics

 * This trope happens often in Scandinavia and The World. Denmark always forgets about the Faroe Islands, his room mate, who owns him money. Also, Norway is apparently oblivious to Kven, even if the guy is right in front of him.
 * In Seventy Seas's side-story is centered around a "forgettable" character. During a bid for attention, he committed burglary and was arrested, even doing that doesn't work, and to top it all off, he's released. Nevermind that he ''drops' a stolen diamond in front of the police.