Meteor Methuselah

Rain Jewlitt, known as the Methuselah, is a spaced-out, immortal, 624-year-old man dressed in priest's clothes. He's got eternal youth and a large price on his head... put there by those questing after the secret of his immortality.

Machika Balfaltin is a tomboyish, energetic 14-year-old girl who, upon the death of her grandfather, sets out to kill Methuselah, the one bounty her assassin grandfather, Zol the Grim Reaper, couldn't kill. Instead, however, Machika ends up being saved by Rain again and again from other aspiring bounty hunters attacking them both.

(Source: Tokyopop)