There's been a number of games about Superman, where he was effectively invincible, but you still could lose when Metropolis ran out of 'health' taking building damage or NPC damage.
Wherever his soul is, Ramus is currently immortal in a way. He dropped to zero and thanke to Hassan became what he is now. I think after he gets killed again it's only a matter of time before he gets back on his feet. If his body is destroyed he'll probably be left clinging on to the remains as an angry spirit or an ugly annoying zombie.
If he wants immortality, he needs to protect his body from damage and decay at all costs. Not sure if his soul will help in that. I rather see him moving on immediately after his hun and po are reunited. His body is dead afterall, so without some form of magic his soul, once freed from whatever holds it will rejoin the Fountain.
If he finds magic that will tie the soul to his decaying body and helps preserve it, he's got a shot at immortality. In that case I'd implement some form of mechanic, where his soul is literally bound to a place of power, like a spring deep in some forest, and once he's down to zero HP a drop from that spring will rejuvenate him. Should the spring be disturbed, he will immiediately be affected in some way.
One other way is to tie him to the immortals or the Tower and use a 'move' mechanic that goes along the lines: whenever you fulfil the Tower's bidding hold +1 (Up to max 3). Use your hold once death claimed you for 24h to rejuvenate your body and return from the dead. If you die with 0 hold your soul returns into the cycle of reincarnation. Roll a new character.
Another way is use another player character (or possibly all of them) as a tether to this world, allowing the dead (<0hp) Ramus to borrow their health to get back on some knd of death roll. That would totally complicate the party's already complicated dynamic.
As symplistic as these propositions are, they would all make for some good storytelling, a I am pretty sure Adam can cook up some even better mechanic if he decides to entertain us with the possibility of an immortal Ramus.