i think its odysseus. odysseus is of course very famous as a greek myth, and also think about, if we have a boss that references the prime soul in every chapter like corpse of king minos or the sisyphean insurectionist, what does the earthmover look like? a centaur yes, but also a horse, what was odysseus famous for building? the trojan horse, AND what did odysseus kill? A GIANT (or i guess a cyclops?). we also have a reference to the odyssey in ship of fools (even if its not in the same chapter) in death odyssey, the name of the song. and also because odysseus WAS in dante's inferno, even if he was still alive then. also odysseus sounds like the kind of guy to have so much will power to become a prime soul. odysseus also can have vengeance against gabriel like the other two because the story would go that gabriel killed him for blinding polyphemus and angering poseidon, and also askaping death with his brains every chance he gets.

and if we go by getting a higher security prison every prime sanctum than my theory for the new flesh keeper would probably be flesh alcatraz.