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 tend to run a long time, 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.
- Bleach actually gives us a reason for this. Ichigo and his father 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.
- Spidey 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 Spider-Man supporting character, Ned Leeds, is also suspected of being), becomes a target for the Green Goblin, loses his memory, etc. He's more recently known as Agent Venom. 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.
- Spidey's various adaptations will even apply this to characters who weren't examples in the source material:
- Eddie Brock in The Spectacular Spider-Man as Peter's childhood fried who attends Empire state university, and in Spider-Man: The Animated Series (and the comics) he is a rival reporter to Parker. Several other characters appear in The Animated Series before becoming their alter egos, most notably Norman and Harry Osborn, and the former appears long before his transformation into the Green Goblin.
- 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.
- Wonder Woman (1987): Vanessa Kapatelis was the awkward teenaged daughter of Julia Kapatelis, the archeologist Diana stayed with when she first left Themyscira. She later ended up kidnapped, tortured and forcibly transformed into the new Silver Swan, to make the villain that much more difficult for Diana to face.
- In Frozen II, Queen Iduna, the mother of Elsa and Anna, is of much more importance than she seemed to be in Frozen. Iduna's shawl is cherished by the sisters, but it is not until they meet the Northuldra (Iduna's relatives) in the Enchanted Forest that they realise the spiritual significance of the shawl and of Iduna herself in the magical world of Frozen.
- Samuel Sterns, the character who will eventually become The Leader, appears pre-villainy turn being a secondary character in The Incredible Hulk. Sadly, he has yet to reappear outside tie-in-comics.
- Sam Raimi's Spider-Man Trilogy, like most Spider-Man adaptations, played with this. 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. (However, the movie franchise was rebooted before that could happen. However, Connors does become the Lizard in the first Amazing Spider-Man film.)
- In the Deryni novels by Katherine Kurtz, keep an eye on the extras, especially if they have rank:
- Archbishop Loris has a relatively minor role in Deryni Rising; aside from leading the troops who arrest Morgan for treason and heresy, he's generally shown in tandem with Archbishop Corrigan and has little to do aside from official functions (sitting in Corrigan's support in the Regency Council, aiding in Kelson's coronation). In the next four books, he's a major antagonist.
- Prince Conall Haldane spends a good deal of time at his father Nigel's side in The Chronicles of the Deryni trilogy, and has little to do other than help out, in part because he's only about fourteen at the time. Look out when he grows older though...
- There are other characters who are either not explicitly mentioned as being present at Kelson's coronation in the text of Deryni Rising, or who are mentioned only in passing. They make bigger impressions later in the series, either because the action shifts to their homes (like Duke Jared McLain and Caulay MacArdry, Earl of Transha), or because other characters are absent or dead (such as Nigel's wife Meraude, who fills Jehana's place at court, and to some extent her brother, Saer de Traherne).
- Buffy the Vampire Slayer:
- Introduced as a (subverted) demon of the day, or at most, demon of the month, Spike endured as a character throughout the TV series' seven seasons and the fifth season of Angel. Series creator Joss Whedon has said numerous times that he did not mean for Spike to be anything more than a 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.
- 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.
- Spidey adaptations are the kings of this. Pretty much every member of his Rogues Gallery is developed for a few episodes first or given a connection to him, and every series ended with at least one or two pre-super heroes or villains still in his sphere, sadly not getting that one more season to properly take the stage. It helps that their powers are given a Meta Origin when originally it was unrelated characters having a Freak Lab Accident every month because the Marvel Universe has No OSHA Compliance. In S:TAS, it's hard to become an animal-based hero or villain without having crossed paths with the guy who invented LEGO Genetics (Curt Connors) and the hero who is of course very interested in the science that altered the spider that bit him. In the Amazing movies, Oscorp is the source of everything, so a lot of names from the comics work there.
- Spider-Man: The Animated Series:
- In the source material, Eddie Brock and his grude against Spider-Man was introduced via retcon. Here, he's present as Peter's rival at the Daily Bugle from day one, and his escalating grudge against Spider-Man is a running thread through the entire first season.
- Felicia Hardy is a classmate and possible love interest of Peter's who takes several seasons to become her comics identity of Black Cat. Michael Morbius has several appearances before becoming Morbius the Living Vampire.
- The Spectacular Spider-Man:
- Eddie Brock is given an extensive role as Peter's childhood friend (influenced by his role in Ultimate Spider-Man), before becoming Venom in the first Season Finale.
- It also has Norman and Harry Osborn (as usual), The Sandman, The Rhino, The Shocker, Doctor Octopus, and Curt Connors appear as supporting characters or minor villains before their inevitable darker turns.
- In The Batman, Ethan Bennet is a long-running crusading police officer (and analogue to Harvey Dent). At the end of the first season, he got fired by Chief Rojas for his failure to apprehend Batman (and defending him), the Joker kidnaps him, works him over and accidentally turns him into Clayface.