Beelzebub, also known as the "Lord of the Flies" is a demon who challenges Mike so he can use his body as a "New Vessel" and to test his might.
He doesn't seem entirely evil as he tests how Mike is for fighting and at one time, came to Mikes house to watch Star Trek with him. He even helped fixed a flat tire Mikes truck had. Those are on days he doesn't feel like going after Mike for whatever reason.
He usually tries to convince Mike to be his vessel but they always tend to fail.
Ironically enough, the Radioactive Uber Clan ignores Beelzebub, usually going after a non-demon next to him, much to Beelzebubs humor.