A mutant whose power is luck, which sounds like nothing and turns out to be the most effective thing in the film.
Domino is the best joke in Deadpool 2 because the film takes it seriously. Wade Wilson tells her luck is not a superpower and not cinematic; the rest of the film is a sustained argument that he is wrong, filmed as a chain of accidents that always end well for her.
Zazie Beetz plays her entirely unbothered, which is the correct reading. She is never in danger and she knows it.