Intern: Maybe even less?
Character creator: Oh! I like you!
When a relatively minor or undeveloped character gains a fanbase all out of proportion to their role in the work.
In horse racing, a "dark horse" is one with little or no racing history, i.e. an unknown quantity. Because gamblers would be "in the dark" about its chances to win, they would be less likely to place a bet on it. The phrase entered mainstream culture largely though politics, where a "Dark Horse Victory" describes a situation where a lesser-known candidate does much better than expected in an election.
By extension, an Ensemble Dark Horse is a member of the cast (anything from a non-lead secondary character to a single-appearance Flat Character), who becomes unexpectedly popular with the fandom. Contributing factors include who and where the work's fanbase is, as well as what the other characters are like in comparison. For example, a quirky one-shot can shine when the protagonist is The Everyman, while The Team Normal is more relatable when everyone else can fly and shoot lasers from their hands. At times, relative character flatness actually helps win the base: fans can more easily project themselves into the work through that person if they have no strong characterization to get in the way.
The writers or producers may be tempted to Retool the work's premise to make the Ensemble Dark Horse more prominent. Sometimes this works, but it usually doesn't: moving a supporting character into The Protagonist's position is hard to do well. If the writers "adjust" the character to fit into a conventionally heroic role, this often removes the unconventional traits that gained the character a following in the first place. However, if they don't do this, the traits that were appealing in a secondary character become grating and unpleasant when showcased by the protagonist.
However, it's still good business for a work to feature its popular characters, even if they weren't intended to stick around. Thus these characters are prone to becoming a Recurring Extra or getting A Day in the Limelight (depending on the kind of character). If the dark horse is an antagonist, the writers may decide to have them perform a Heel–Face Turn in situations where the only other option is being killed by the protagonists. However, if the series doesn't have an end planned, it's more likely that they'll just escape.
When the dark horse becomes an important character, they're a Breakout Character. If they have a fanbase despite only appearing in one or two scenes in the entire work, they're also a One-Scene Wonder. See also Adaptational Badass, Ascended Extra, Memetic Bystander, Lower-Deck Episode, A Day in the Limelight, and Unpopular Popular Character. Contrast The Scrappy, a minor character who gains an unexpected hatedom, and the Creator's Pet (the writers like the character, but the viewers do not). Can lead to the Draco in Leather Pants version of Misaimed Fandom, where the minor character's fanbase is doubly unexpected because the author wrote them to be unlikeable. The natural extension of this is the Spotlight-Stealing Squad. Major characters who end up overshadowing their castmates often fall under Face of the Band, whether they're the true lead character or a supporting member who ends up overshadowing the lead, à la Darth Vader or Stewie Griffin.
Compare Cult Classic, for works whose fanbase is significantly larger than the work's quality, budget, or influence would suggest; compare and contrast Base-Breaking Character, where the fanbase splits between those who love the character and those who despise them.
No Real Life Examples, Please! The real world isn't a story with characters that could potentially make it better.
Although this applies to individual characters, as a YMMV item, it should not be listed on character pages. This seems counter-intuitive, but character pages are meant to list tropes audience members can see in the work in question, not fan opinion expressed outside of the work. Also, please do not list characters until a month after their introduction. This is to ensure that the character remains popular in the long run.
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