The main character of Undertale is of unspecified gender, and MatPat was (somewhat clumsily) addressing this by referring to them alternately as "he" or "she", at random. Since the last video, he seems to have responded to criticism by including "they" among the random pronouns. This is pleasantly surprising progress!
@jk the only accurate pronoun is “you”
@actualgarbage The player is a distinct entity from the player character
Generalistic and moderated instance.