Destined Bystander

""The thing about Spider-Man is that his enemies can't just be sociopaths who like pretty things. Some are, sure, but an astonishing number of them are his best friend from high school, or his girlfriend's father, or the cat he raised from a kitten. Something angsty. So there's the Green Goblin, who's not really that inspiring as a villain. I mean, pumpkin bombs? But in his various incarnations the Green Goblin has been practically everyone on Peter Parker's speed dial.""

- Lore Sjoberg, The Book of Ratings

Spoiler Warning!

Do not actively try to become a Sidekick or True Love. If you really have what it takes, you'll wind up with the role no matter what you do. Sometimes, you will be drawn into some new role or destiny by default, be it hero or villain.

Superhero comics are long runners, with large amounts of characters that range from the major to the minor. Thus, there is never a shortage of characters that can dynamically drive the plot, and there's always people who the hero is close to or knows that can become... something else. Sometimes, the guy who was the hero's former roommate, or an old teacher, or a neighbor, can, through what seems like destiny, become intwined into some new role, be it antagonist, love interest, mentor, or otherwise.

This is when a character who first appears as a supporting character eventually turns into something more through Character Development, a Legacy Character. The best friend becomes the Big Bad, the old neighbor becomes the Love Interest, etc. The person who was once on the sidelines is inevitably drawn into the main event. They have a definite place in the future of the story.

If the character was always intended to be important, that's a Chekhov's Gunman. If the character was hiding their identity during their first appearance, you have a Red Herring Shirt. If a character in an adaptation is introduced earlier than in the source work, that's an Early-Bird Cameo.

Anime and Manga

 * Bleach actually gives us a reason for this. Ichigo  is causing the latent abilities of his friends to manifest. Only Orihime and Chad are anywhere near the power level of Ichigo, but Tatsuki, Keigo and Chizuru are given a filler episode two-partner to show off their powers beginning to appear, and even Ichigo's sisters are caught up in it.

Comic Books

 * Spider-Man owns this trope in both the comics and all adaptations, it's part of what makes reading the comics (where the writers probably tried to used named characters as much as often) and watching the shows so fun... most named characters have a destiny interlocked with the hero, either badly, or not, but they all have lives and roles before their inevitable (and, part of what makes it so sad is that it is, ultimately, inevitable) destiny strikes...
 * A few notable examples from the comics: Nearly everyone named that Peter Parker went to high-school with has had deeper involvement in his story as the years passed. Liz Allen (whose brother turns out to be the Molten Man) fell in love with Harry Osborn, whose father turned out to be the Green Goblin, and who eventually went mad and took up his father's title. Flash Thompson, Peter's bully-turned-friend, is suspected of being the Hobgoblin (whom another long time Spiderman supporting character, Ned Leeds, is also suspected of being), becomes a target for the Green Goblin, loses him memory, etc. Even one or two appearance character Sally Avril decides to become a heroine named Bluebird (and subsequently dies). It's a big case of writers deciding to use known characters instead of spontaneously making new ones, but it often looks like just being near Peter Parker is an invitation to be drawn in.
 * This is also often used in adaptations:
 * Eddie Brock in The Spectacular Spiderman as a fellow student to Peter, and in Spider-Man: The Animated Series (and the comics) he is a Rival Reporter to Parker. Several other characters appear in Spiderman The Animated Series before becoming their alter egoes, most notably Norman and Harry Osborn, and the former appears long before his transformation into Spiderman's archenemy: The Green Goblin. Other examples include Felicia Hardy, who takes several seasons to become her comics identity of The Black Cat, and Michael Morbius, who has several appearances before becoming Morbius the Living Vampire.
 * In The Batman, Ethan Bennet is a long running crusading police officer (and analogue to Harvey Dent). At the end of the first season, the Joker kidnaps him, works him over and accidentally turns him into Clayface.
 * Likewise, The Spectacular Spider-Man, as noted before, not only gives Eddie Brock an extensive role (influenced by his role in Ultimate Spider-Man), but also has Norman and Harry Osborn, The Sandman, The Rhino, The Shocker, Doctor Octopus, Curt Connors, and, possibly,, as supporting characters or minor villains before their inevitable darker turns, as well as, as said before, every named schoolmate Peter had in the comics, as well as (again, as noted above) Gwen Stacy, whose fate is well known.
 * The recent movies have been playing with this as well. Harry Osborn appears as one of the main supporting characters for the first two movies before moving on into a temporary villainous role as The New Goblin in the third. Curt Connors appears often throughout the second and third movies, before perhaps becoming The Lizard,
 * Daniel Sterns, the character who will eventually become The Leader, appears pre-villainy turn being a secondary character in The Incredible Hulk.
 * Jimmy Olsen was originally made up for the Superman radio show, so that Supes could have someone to discuss the plot with. Over half a century later, radio has all but faded away, yet Jimmy remains as Superman's Best Friend, and an inextricable part of the core cast. He was even the star of his own comic book series, for a while.

Live-Action TV

 * Introduced as a (subverted) demon of the day; or at most, demon of the month; Spike of Buffy the Vampire Slayer fame endured as a character throughout the TV series' seven seasons . Series creator Joss Whedon has said numerous times that he did not mean for Spike to be anything more than minor character and planned to kill him off not long after his first appearance.
 * Anya was also initially just the throwaway villain of an It's a Wonderful Plot episode, which ended with her alive but without her powers. When said episode proved extremely popular and the creators decided to revisit it, Anya was brought back, after which she made a few more minor appearances before being reworked as a Scooby proper and Xander's love interest.

Tabletop Games

 * In White Wolf's roleplaying game Scion, characters who exercise their proto-godly nature around humans have a chance to Fatebind those humans to the scions themselves, essentially forcing them to serve as potential love interests (regardless of whether either party in the matter is actually attracted to the other), rivals, accidental victims, or pawns in the scions' enemies' schemes.

Web Comics

 * Leslie Bean of Shortpacked! showed up as a random grocery store employee before Willis decided to reuse her character design, therefore making her a prominent character.
 * In the Ciem Webcomic Series of 2005, Dolly Malestrom was S1 the Bunny and otherwise Jeraime's wife, and aware that Candi is Ciem. Dolly played no role afterward. By the 2007 version, there were plans to have her become The Earwig, later made official as part of the plans for Ciem 3. The Comprehensive Gerosha timeline does away with S1 entirely, and has it that she's destined to be The Earwig by Condemnation.
 * Miriam in 2005 was originally just Candi's sister, and nothing special. The first time Ciem 2 was tried, her Sniperbadger persona as a hobby was retconned so that Candi would have a personal reason to wear a centipede outfit again and fight Aard. Since then, Miriam's Sniperbadger hobbies have become more integral. All future versions of the story make Miriam's hobbies more narrative-important.

Web Original

 * As Stationery Voyagers was in development, Nika Inkista, went from being a minor cameo in Season One to suddenly becoming important by the end of Season Two, if quickly forgotten by the main episodes after fulfilling her role. Licorine and Stella-Marie also learn that they Can't Stay Normal once the Yehtzig League forces itself on them.