Chara is genderless, but this is where headcanons come in: some think of them as a girl, others as a boy, and others as ambiguous. There's no right 或者 wrong, so feel free to choose whatever 你 like. ^^
According to the undertale wiki, "Chara looks strikingly similar to the protagonist, down to a "similar fashion sense," as 说 由 Asriel, and just like the protagonist, has an ambiguous gender."