Forgotten Friend, New Foe

""Every murderer is probably somebody's old friend.""

- Agatha Christie, The Mysterious Affair At Styles

The shining gem of the Non Serial Movie, an antagonist who is inevitably related (especially romantically) in some way to one of the characters. The trouble occurs when they revisit, and the current situation (say, a character's new Love Interest) is one that doesn't suit them at all. For maximum effect, The Reveal pulled off near the very end gives a little twist to the "villain" as well as some surprising insight to a relevant character. These work well for such a movie because it is much easier to add Backstory without affecting things later, and is often never mentioned again in the television series proper.

Subtrope of Childhood Friends. Compare Beleaguered Childhood Friend. Contrast Forgotten First Meeting.

Anime and Manga

 * Fiore in the first Sailor Moon movie, which also explained how Usagi and Mamoru first met.
 * For those not familiar, a 10-year-old Mamoru was in the hospital after being in a car crash that his parents died in. Fiore is an alien disguised as a human who was ill because he couldn't survive on Earth at the time (though he adapts). 4-year-old Usagi is there to visit her mother who is giving birth to her younger brother Shingo, and she sneaks into Mamoru's room. Usagi and Mamoru talk for a bit, then Usagi gives Mamoru one of the roses she had originally brought to give to Ikuko-mama. Mamoru then gives that same rose to Fiore as he leaves Earth.
 * Forms the basis of one of the Urusei Yatsura movies: Ataru made a Childhood Marriage Promise to a girl he frequently saw on the playground. Fast-forward ten years, and we discover that not only was she an alien princess (with an entire army at her disposal), she's never realized that the proposal was meant as a joke. Understandably, Lum isn't appreciative of this state of affairs; neither is Ataru, but that's because he isn't getting any tail out of it.
 * Ukyou first appeared in Ranma One Half in this role, but did a Heel Face Turn (such as it was) and put herself in the running for Ranma.
 * Sara's long-lost brother Ralph in Soukou no Strain is revealed near the end of the first episode, after the Union has been praising him and Sara herself chasing after him for years, to be the most fearsome villain the series had to offer.
 * The demonic Duel Monsters card Yubel in Yu-Gi-Oh GX was revealed to have been owned by Juudai back in the day. He didn't forget her on purpose, though, having his memory suppressed due to the nightmares he suffered when she was in pain up in space.
 * In Code Geass's Backstory, they were best friends, but eight years later, Suzaku Kururugi and Lelouch Lamperouge are on opposite sides. Neither is an outright villain... nor are they full-blooded heroes.
 * Very similar situation in Gundam Seed, which has Athrun Zala and Kira Yamato fighting each other despite having been the best of friends earlier in life (something which causes both massive amounts of Angst).
 * Koujirou Saeki from The Prince of Tennis, in regards to the Fuji brothers.
 * Kenichi the Mightiest Disciple has, who was one of Kenichi's childhood friends.
 * It doesn't fit them all that well, but both of Hayate the Combat Butler had been acquainted with Hayate in the past.
 * Mendou in White Album has aspects of this, only she is a benevolent, and not a hostile figure (as far as these things can be counted)
 * Love Hina plays with this trope. Keitaro knows about his childhood friend whom he promised to get into Tokyo University with...he just can't remember her name or face.
 * In Shoujo Kakumei Utena, we find out pretty early on in the series that Utena met Touga and Saionji when she was young. Shortly after that, she met Dios. Shortly after that....
 * Justified in that she only saw the boys once before the series and probably wasn't even paying attention, in the case of Anthy she was given Laser-Guided Amnesia.
 * In Ghost in The Shell Stand Alone Complex is this to the Major.
 * A variation in Pokémon Special.  both remember having a childhood friend, but they don't remember what the other looks like. As such, they don't recognize each other when they reunite five years later and get off the wrong foot. Justified by the fact that they both had their personalities completely flipped since they were kids.
 * Makarov, the Big Good of Fairy Tail and master of the titular guild, recognizes Grimoire Heart's master Hades as.
 * Shiroemon in Dorabase, who befriended Kuro during their days in school. They broke off the friendship after Shiro refuses to stay as he is (being good at nothing). From what is known from the flashbacks, he believes that nobody can be counted on to succeed by one's self after what he's been through. He is initially presented as a Jerkass, selfish character who openly shows disrespect towards his teammates. However Character Development kicks in soon enough and he gets better, and his relationship with Kuro being more rivalry than dislike, but he is still in the Whiters, Dora's Worthy Opponent Arch Enemy.

Live Action TV

 * In the season 2 episode of Buffy the Vampire Slayer 'Lie To Me', Billy Fordham--"Ford"--an old friend of Buffy's from LA, shows up in Sunnydale. He has figured out she's a Slayer... and that's just for starters.
 * Doctor Who has Mels, who grew up with Amy and Rory as their best friend, but whom the Doctor (and audience) have never met before. After she explains she skipped appearing at their wedding, she pulls a gun on the Doctor in order to "Let's Kill Hitler!" but gets fatally shot during the escapade. This is where it gets complicated....

Comic Books

 * Tommy Elliot / Hush from Batman - a hitherto-unmentioned childhood friend of Bruce Wayne's who turns out to be the villain behind the arc introducing him.

Tabletop Games

 * Magic the Gathering has Gerrard, hero of the Weatherlight saga, pitted against Volrath, evil evincar of Rath and Gerrard's jealous adoptive brother from his youth.

Video Games

 * Misha of the first Ar Tonelico game spends a good half the game trying to convince Lyner that she and he grew up together. It takes most of the game before he remembers - and it's up to the player's choice eight hours ago whether or not she's the Unlucky Childhood Friend or Victorious Childhood Friend (barring going for the secret Love Interest...)
 * Played with in Kingdom Hearts: Chain of Memories. Naminé is forced to switch Sora's memories of Kairi for some of herself, which makes him think that Naminé was his Forgotten Childhood Friend.
 * Rule of Rose,
 * More literally,
 * In Epic Mickey, Oswald the Lucky Rabbit is Mickey Mouse's older half-brother who was lost to Walt Disney over a legal dispute and replaced by Mickey himself. Judging from what information we have on the game now, it's not going to be pretty when he and Mickey first meet.
 * Persona 2: Innocent Sin
 * Used in Final Fantasy VIII when it's revealed that all of the SEED members in the main party had grown up together in the same orphanage, but most of them had forgotten. A Justified Trope, as amnesia is an established side effect of letting Guardian Forces take up space in your brain.
 * Shiki doesn't remember, the Big Bad in Tsukihime, due to some Laser-Guided Amnesia. To a lesser (or greater since he never really does remember them) extent he also forgets about Kohaku and Hisui, and in the routes he does remember them, he cannot recall their names and also gets them confused
 * Soul Nomad and The World Eaters state that, one of the villains was this to , with this being a big part of the former's Start of Darkness. This plays a part in
 * Blaz Blue Continuum Shift reveals that  met each other as little kids.   probably forgot due to being preoccupied with the whole   and.

Visual Novels

 * and in their respective routes in Tsukihime. And maybe, too.
 * In the Clannad Visual Novel, Kotomi's route ends up with her being the Victorious Childhood Friend.

Western Animation

 * In The Venture Brothers, Rusty doesn't remember Hector Molina, his best friend throughout his childhood, at all when he finds him in the Compound.
 * Even though Hector was working for him. Then he fires him.