he can be
yeah-so basically-
No one can see him unless they are the next person he's meant to make a deal with.
He is required to make a deal with that person-if they refuse, he's just gotta keep trying.
Once he makes the deal, he has to stay with said person until they die.
He's not allowed to fall for mortals or he will lose his immortality
Upon getting a certain number of souls his full powers and ability to pass between hell and earth are restored