Beelzebub (ベルゼブブ Beruzebubu) is a fallen angel who was over-confident of his abilities and went against God. As his punishment, he was cast into hell, where he turned into a demon. It seems to be power hungry, literally. It eats the abilities of power users with a flame that burns at 1500° Celsius. Heike reveals in chapter 138 that as one of the founders of Eden, he was the one who took away Ogami's ability to control Beelzebub, in the form as a giant feather. Beelzebub's true form is the achromatic flame of purgatory, Seraphim, the six winged angel.
In chapter 161, Ogami call out Beelzebub to fight the "Cell stimulation" power users but Beelzebub end up eating him. With the shock, Toki and Sakura thought Ogami still couldn't control Beelzebub. But then Ogami reaapered in a reaper form. In chapter 162, Heike state that "The true form of Beelzebub has woken up".