Known as Beelzebub or Baby Beel (ベル坊Beru Bō?). He is the youngest son of the Devil King. He is very attached to Oga, always clinging onto Oga's back or sitting on Oga's head. When he becomes agitated (and initially when he saw blood), he'll often throw a tantrum that will spontaneously electrocute anyone within his vicinity. Without Oga, he is physically weaker than even the average human child, but contact with Oga allows him to unleash his demonic powers. After fearing for Oga's safety when he contracted a fever, (as his power would end up killing Oga) he severed the contract and ran away. He then became somewhat attached to Toujou, who resembles Oga (at least in his own mind). Later, when Oga and Toujou start to fight, Beel snaps out of his fever and goes back to Oga. At that point, their mutual trust becomes so great that Beel even erases the Zebub spell to allow Oga to fight Toujou without using demonic powers.
Beelzebub's characteristic cry is "Daabū!" and he is always naked with a pacifier in his mouth. He, like Oga, is shown to anger easily, yet his outbursts are portrayed in a more comical fashion. He, like other demons, is able to sense when a human is particularly strong, and this first leads him to take a liking to Kunieda. Later, when she learns how to use demonic powers, Beel becomes even more attached to her. He has also shown to have some liking to Toujou, who appears to remind him of Oga. When Oga reunites with Miki, Beel showed immediate disgust and possible fear toward the Saint Ishiyama Student. The reason for this has not been stated.
In the demon world, the extent of Beel's true power is finally known when he strikes down a gigantic monster bird in one hit and later defeats a monster larger than a mountain under Oga's direction. Through the series (so far covering a period of about six months), he shows some development, as expected from an infant, and his ability to interact with other people increases. He and Kunieda's little brother Kouta (see below) develop a rivalry, with comic effect.
His name is borrowed from a semitic deity that was incorporated as a demon by Abrahamic religions.