Immortal Rain Vol 1

Immortal Rain by Kaori Ozaki. A Fantasy manga rated Older Teen, published in English by Tokyopop.

Rain Methuselah has lived for centuries with a massive bounty on his head, because whoever discovers the secret of his immortality could win eternal life. When Machika, the granddaughter of a bounty hunter, sets out to claim the prize, she finds a weary man who wants nothing more than to finally die.