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From Wikipedia:

A young female assassin named Machika swears to kill the one bounty her grandfather couldn't get: the immortal Methuselah. But when she goes after him, she finds a clumsy, kind man named Rain who has lost more than he can bear in his unnatural lifetime. Unwilling to admit at first that she likes him, Machika willingly follows him as he waits for the one who made him immortal, still swearing to take his life one day. Their love for each other grows as they battle bounty hunters, a mysterious organization, and angels in order to save Rain and perhaps even humanity.

From Tokyopop:

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.

Sound like the beginnings of an odd relationship? You won't believe where their journey takes them ...

• Immortal Rain: Shinigami of the East (Side Story)
• Immortal Rain: Shinigami of the East (Prequel)