Ybgir

The Jinx Monster is a interdimensional demon appears in Jinx, after Rigby does a forbidden 'Jinx cheat code' in the bathroom. The Jinx Monster 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. The Jinx Monster has an ability similar to werewolves as it scratches its victims to turn them into other jinx demons. It is also suggested that his name is Ybgir, because they must say the monster's name three times to send it back where it came from, not to be confused with the Ybgir of Dizzy World.

Appearance
The Jinx Monster 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 the dark way. Either way, the Jinx Monster has purple eyes, black skin/fur, and white outlines, giving the appeal that the monster is in a negative art style.