Etrigan

''Gone! Gone! -- the form of man -- Rise, the Demon Etrigan! ''

Etrigan is a demon of Hell bonded to a man named Jason Blood, who usually acts on the side of good despite his violent tendencies. They were bonded together in Camelot by the wizard Merlin as a punishment, imprisoning Etrigan and making Blood effectively immortal. Despite sharing the same body, they are usually at odds with each other and Blood retains a lot of guilt based on Etrigan's demonic actions. Etrigan's most distinctive trait is his skill at speaking in rhyme, a symbol of his social status in Hell's aristocracy. He is the leader of the Demon Knights and has been involved with the Justice League. The Demon was created by Jack Kirby, first appearing in Demon #1. (1972)

History
Etrigan was a rhyming demon in the service of Lucifer. Not content with his life, he attempted a rebellion, which did not succeed. For his insolence and betrayal, he was banished to Belial's son Merlin, who had asked Hell for a means to protect Camelot. He fused Etrigan to the rebellious scribe Jason of Norwich, and they would walk Earth together, cursed to live with each other.