In HetaOni, Ryuuzuu is a former victim of the mansion. Like Italy, he entered with a group of friends and lost all of them before finally getting out. He was given the same deal: if you get out with your friends, you win. If all of you die, I win. Ryuuzuu accepted the deal and went back.
However, he never succeeded in saving his friends. Ryuuzuu went mad. He eventually went to the beginning of a new loop and automatically killed his friends. The Oni showed and saw the scene. Intrigued, the Oni he’d Ryuuzuu live in the mansion.
Ryuuzuu believes he cheated a win and is in charge of the Oni. He’s been trying to beat Italy in this “twisted game”.
Headcanon shared between me and marine-c-jones