Fan Casting Ensemble Dark Horse in Sort by Tropes

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.