Known as Beelzebub or Baby Beel (ベル坊 Beru Bō).
He is the youngest son of the Devil King and is very attached to Oga; always clinging onto Oga's back or sitting on his head.

Beelzebub's characteristic cry is "Daabū!". When he becomes agitated, 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, however, contact with Oga allows him to unleash his demonic powers. Beel's power increases as he ages and so does Oga's, but the process is accelerated when both undergo training to allow Oga to control his possession by Beel.

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 develop a rivalry with comic effect.

Source:Wikipedia