Raizen is one of the three rulers of Makai. He first appears in the show when Yusuke is fighting Sensui and he interrupts and possesses Yusuke. When Raizen's men retreive Yusuke from Earth he tells Yusuke that he is his ancestor. Yusuke then trains to defeat his ancestor Raizen in battle.

During their battle Raizens past unfolds. He tells Yusuke how he would raid villages and eat human flesh. But when he once was in battle he became wounded. A female human doctor helps him and they fall in love. He then promises her that he would never eat human flesh again, with this he is slowly dying over the course of the years since he last ate human flesh. With his passing he leaves Yusuke to take over his position.

In episode 107, one of Raizen's fighters says he had fought Raizen himself over 1,500 years ago and it was just childs play for Raizen, and Raizen states he trained for 1,000 to reach he power level, so that should make Raizen be at least 3,000 years old.

In episode 111, as Yusuke leaves Demon World he goes to his father's burial and talks to him some, when he is departing he says "Seeya dad". Also, the men that train Yusuke say "you don't want to disapoint your Ancestoral Father", so it's pretty confusing as to whether or not Raizen is Yusuke's dad or just ancestor.

INFORMATION FROM ABSOLUTEANIME.COM