Here's my pitch: a centuries-old swordsman bound to an Eldritch Arm that imbues him with immortality. Wanders the world sort of like [i]Yojimbo[/i] as a nameless vagabond, though meddles with the affairs of common people when he feels need to.

Now, I didn't see an explicit Japan analogue written up, so I'll refer to my character as a "swordsman" for now, but my original idea had him as basically a ronin.