A minor character who is given a greatly expanded role in the later part of the series, a new adaptation of a story, or in the Expanded Universe. It is usually safer to do this with an otherwise obscure character without much depth, since the likelihood of creating plot contradictions is lower. On the other hand, the character may or may not be allowed to have a huge effect on the plot depending on how much the writer is trying to follow source material. Can lead to some bizarre incongruities in planning (e.g. Overtook the Manga, Restricted Expanded Universe). Sometimes the minor character becomes an Ascended Extra just so the writers can immediately turn them into a Sacrificial Lion.
This naturally occurs as a series goes on for long periods of time and useful for filler, technically not requiring you invent someone whole cloth. Be warned however that this character might be more liked by the fandom than the writers, who may have no compunction with getting rid of them when convenient.
This also happens somewhat regularly in Fan Fiction, often when writers feel a character has potential that wasn't fully exploited in the original source. See also OC Stand In, They Wasted a Perfectly Good Character.
Compare the Breakout Character, where a supporting character gains a fandom that rivals or exceeds the main characters and thus becomes more prominent in canon. Compare also the Ensemble Dark Horse where in a similar fashion, a supporting character role remains the same but that character receives more fans . See Breakout Villain specifically for when a would-be one-shot villain ascends to Big Bad status, either through Ascended Extra or Breakout Character. A One-Scene Wonder is not an Ascended Extra, at least officially; fanfic, on the other hand...
See Mauve Shirt for when a member of the Redshirt Army gets his own name and minor characterization, but still doesn't become more than a minor supporting character in any way. For the villain equivalent, see Mook Promotion. The inverse of this trope is Demoted to Extra. Retroactive Recognition is when this happens in Real Life.
Compare Red Herring Shirt, only the ascension is intended from the beginning, and Ensemble Dark Horse, where fans become fascinated by a character who canonically has only a very small role, at least initially.
Also, contrast with Demoted to Extra.
In his first appearance in the Robin Hood ballads, Guy Of Guisborne is a bounty hunter in horsehide armour who Robin beheads. Modern retellings have promoted the character being The Dragon to the Sheriff of Nottingham (or sometimes the Big Bad himself) because he was one of the few villains from the ballads to be given a name (and a catchy one, at that), and the fact that held his own against Robin in an epic swordfight described as lasting "two howres".
All the time. Go through a list of your favorite bands, and it's almost a guarantee that at least a handful of those bands had a touring guitarist/bassist/drummer that eventually became a full-time member after a year or two.
Opening Acts. It's not uncommon for them to become more popular than the band they toured with. Or never heard from again.
Rare musical instrument example: Keyboard instruments (usually the harpsichord or organ) were originally used just for bass accompaniment (basso continuo). Then along came Johann Sebastian Bach, with his keyboard toccatas, preludes, fugues, passacaglias, fantasies, etc. which showed the world that keyboard instruments were fit as the main instrument of a concerto or even as a solo instrument.
The harpsichord fell out of favour in the 19th Century, with Bach music largely having been migrated to the piano. In the 20th century, it enjoyed a revival, thanks largely to keyboard virtuosi and harpsichord enthusiasts Violet Gordon-Woodhouse and Wanda Landowska.
This happens quite a bit in wrestling. Many times when there's a need for non-wrestling roles in a show, they'll hire local wrestlers or talents from their developmental programs to fill in, and it stands to reason that eventually a few of them will get called up. A couple of famous examples:
And there's a really cool one, which counts as a Retroactive Recognition moment if (like me) you only started watching wrestling in 2006. Remember at the 2005 Royal Rumble when Kurt Angle was so incensed by Shawn Michaels quickly eliminating him that he got back in the ring and returned the favor? See all those referees and other backstage personnel coming out to stop Angle from beating Michaels to death? Isn't one of them....? Yup, that's Finlay. Of course, if you were just a casual WWE fan you had no idea that he'd been in WCW a decade earlier and had been training the WWE Divas for several years.
One of the security guards who carried a terrified Simon Dean to the ring for his match against the debuting Boogeyman on the December 2, 2005 SmackDown!? That's Cliff Compton, later known as Deuce of Deuce N' Domino.
The Hamish And Andy show has Cackling Jack, Who allegedly started out on work experience is now pretty much the third member of the comedy duo. though there are lile several behind the sense guys who are more important then him, he now probibly appears on mike more then any one else other then Hamish and Andy.
The Shadow started out as the narrator of Detective Story Hour. When audiences proved more interested in the Shadow than the stories, writer Walter Gibson was commissioned to write about him. A full radio show started shortly thereafter, and a legend was born.
Helmie was just supposed to be an unimportant barge helmsman who got Hotwire to Ogel's Island, but his character ended up being developed much more than Jackson Lake had initially expected.
Hertz was introduced as a Red Shirt, but when he survived longer than usual, he graduated to Mauve Shirt and, ultimately, became that guy from that show's new primary character.
Frank Einstein was a seemingly minor scientist introduced shortly before the Goo Caverns Story Arc, who went on to become Dino Attack Team's Reverse Mole, the creator of Pterisa, and the one responsible for Rex's Laser-Guided Amnesia.
Like Hertz, Zelda Frodongan was introduced as a Red Shirt who defied the odds and survived, allowing her to become an important cast member.
The alpha female T-Rex made her brief, minor debut in Alpha Team: Mission Deep Freeze RPG. She was expanded upon and given much more character in Dino Attack RPG.
Toku from Legend of the Five Rings. A simple peasant farmer who became a ronin samurai, he was originally a one-shot character, but he was so loved by the fans of the card game that he was worked in as a seminal part to the overarching story, including having a Clan founded for him (the Monkey) and eventually dying in a Heroic Sacrifice...then serving in the Army of the Dead and being named a minor Fortune (deity) by the Emperor.
In Dungeons & Dragons, the Formorians were formerly just a very ugly type of giant that rarely got used. But with the creation of the default Points Of Light setting for 4th Edition, they are now the major villain race of an entire plane (Specifically, the Feywild).
In A Touch Of Evil, Lucy Hanbrook and the Scarlett Shadow were originally available as Allies. The Hero Pack One expansion made them into fully playable characters.
In Warhammer 40,000, Harald Deathwolf was simply one of the 12 Wolf Lords of the Space Wolves with no backstory. Then the Codex author thought the generic Wolf Lord on Thunderwolf model looked too cool not to have its own backstory, and so wrote the full Saga of Harald Deathwolf in White Dwarf. The model even comes with a moulded shoulder pad allowing you to represent him specifically, and he was one of two Wolf Lords to get a transfer sheet for their Great Company. (Ragnar Blackmane, an actual special character didn't) Bear in mind, he's not even an official named character.
Several minor characters in Shakespeare's Macbeth seem to barely qualify to have names until very late into the play.
Mercutio in Romeo and Juliet. In Shakespeare's source, The Tragical History of Romeus and Juliet, he makes one appearance, and only gets mentioned at all because he happens to be sitting next to Juliet at the feast. Shakespeare makes him Romeo's best friend, the only person who can maintain connections with both the Capulets and the Montegues, a Trickster and a Deadpan Snarker.
Will Parker is one half of the Beta Couple in the musical Oklahoma!. He wasn't really a character in Green Grow The Lilacs (the straight play Oklahoma! was derived from), which mentioned him once.
Neil Simon's first play, Come Blow Your Horn, briefly mentions an offstage character named Felix Ungar. Felix, of course, later became half of the eponymous duo in Simon's The Odd Couple and its film and TV adaptations.
In the stage musical of Beauty and the Beast, the feather duster and the wardrobe are given bigger roles, more developed personalities, and names, Babette and Madame de la Grande Bouche, respectively.
In-universe example: Christine Daae in The Phantom of the Opera was a chorus girl until Carlotta stormed off the set in a huff.
Speaking of which, many of the alternates/understudies for the three main roles eventually took on the role proper in other productions—Rebecca Caine was an alternate for Christine in the initial London production before being the main Christine in the Toronto one, many of the actors who played Raoul eventually played the Phantom, etc.
Miss Saigon: Ruthie Henshall is one of the nameless bar girls in the original London production before eventually playing the role of Ellen (the woman Chris marries after the war). In fact, most of the "bar girl" actresses went on to play Kim (the titular character) or Ellen.
The Toa Inika from BIONICLE started out as mere Matoran who were barely mentioned (if at all) in the Chronicles series of books. In Legends, however, they become the central heroes for the first two thirds of the Mask of Life story arc.
Take Care Bear of the Care Bears had only appeared in one coloring book in 1987, and nowhere else. Most fans never knew her. Then, the 2002 relaunch of the Care Bears included Take Care Bear in the toyline. Her tummy symbol from 1987 was an apple, because Take Care Bear promoted good health; but the apple now belongs to Smart Heart Bear, so Take Care Bear's tummy symbol is now a heart hugging a star.
After being Demoted to Extra in the original version of Tsukihime, Satsuki Yumizuka (Sacchin to fans) has been confirmed to have regained her own route in the upcoming Tsukihime remake. Isn't it great, Sacchin?
While the remake appears to be in Development Hell, that hasn't stopped Satsuki from appearing as a playable character in the spinoff game Melty Blood.
While an important character, Archer has little actual relevance to the Fate route that the anime is based off of. However, he's such a popular/interesting character that his importance and screen time are rather increased. They mostly drag in bits of information from the UBW path and fully animate the battle with Berserker, which was only alluded to in the game itself.
Sakura's rare and unobtrusive appearances in the first two arcs would not lead players to suspect she'd become the Love Interest and MacGuffin Girl of the final arc.
Ethan, a minor character in Season 1, got a rather surprising Promotion to Opening Titles in Season 2. However, this didn't end up translating into more screentime, and the only episode where he ever really got any prominence was the one in which he died.
Brent, a wordless extra in of one episode in Season 1, gained so much momentum with a guest stint in Season 2 that he's made it to the title credits of Season 3.
Mr. Livingston was originally Turk's Bad Boss, but he ended up reprising the role in Season 2 several times due to how he was tied to the background stories of several characters. Also Promoted to Opening Titles in Season 3.