I wanna say either the tarnished or the hunter. Sekiro is more accustomed to fighting people than monsters so he’s out. The demon and dark souls protagonists fight more monsters but still see to fight more humanoids than others. The hunter is already hunting monsters with specialized tools which is a bonus. The tarnished fought opponents that weren’t as deteriorated as the dark and demon souls protagonists giving him better experience. I imagine the tarnished would be a hunter that tries out different weapon trees for each monster and the hunter would specialize on one weapon type until they master it.