Jekyll no Tsukaima

Jekyll no Tsukaima summary:

Densuke Konomiya is a powerful exorcist, hunted down by numerous Magical Beings who hope to drink his blood and gain immense power. He can normally get rid of them without much effort, until he meets one who won't give up no matter how many times he's beaten down. Instead of fighting him again and again, Densuke decides to make the pest his familiar instead.