Like in Absolutely Ruined Love. But he has 2 personalities, here is 4
I agree he probably has DID (dissociative identity disorder) and even though it's rare, it might be that this fourth persona that appeared is actually the host (or main personality) that due to the trauma that caused the split, got pushed in the ""back seat "" and has never resurfaced afterwards cause he or the others felt like it wasn't safe enough. It is (even if in an exaggerated way) kinda of showed in the movie "Split" where the host couldn't handle the situation and got pushed back/ hidden by the other personalities that took over the body, the only question now is if this "new" Mugo Is gonna be good and loving or a bitch and also what plans the author has for the others
i think its some kind of split personality disorder not like three souls getting stuck but kinda seems like split personality due the things happends in the past- AND SH HOLY PEAK