Kanon

Kanon was the legendary human Hero who fought against the Demon King Anos during the Great War that occurred 2,000 years ago. Kanon was famous for having seven sources that made him nigh-immortal and the wielder of the legendary holy sword Evansmana, the Sword of Three Races.

[Spoiler] With his remaining source, Kanon reincarnated repeatedly and eventually became a pure-blooded demon: Lay Glanzudlii. [/Spoiler]

(Source: Maou Gakuin Wiki)