You should definitely give him some sort of immortality like the homunculus from full metal alchemist. Maybe even give some abilities from that show as well?
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if you want to you can take a few aspirations from SCP 049 that way he has an offensive abillity(touch of death) which is already a spell on the anime and a summons ability thanks to SCP 049-A aka necromancers ability to summon's zombies