Ybgir seems to be the exact opposite of Rigby, as he is taller and more muscular. But it could be that it has no true form, only transforming into the being that uses the forbidden jinx cheat code, such as Rigby.
Ybgir has an ability similar to that of werewolves, as it scratches its victims to turn them into other negative demons, although its power had no effect other than amnesia upon dying/returning to the mirror dimension. It is also suggested that its name is Ybgir, because they must say the monster's name three times to send it back where it came from, but it may only be that way for Ybgir and may be the reverse name of whoever the jinx cheater is, not to be confused with the Ybgir in Dizzy World In "Dizzy".
Ybgir is a monster from another dimension from behind the bathroom mirror. In Jinx, it looks like a monstrous, muscular Rigby, though this cannot be its true form as it will become whatever species the person trying to break the jinx is. Either way, a negative monster has purple eyes, black skin/fur, and white outlines, giving the appeal that the monster is in a negative art style.