Kōta Kunieda

Kōta is Aoi Kunieda's younger brother. Kōta is the reason Aoi created her alter ego, Aoi Kunie, so that nobody sees her taking care of a baby, however the mothers at the park recognize her and believe Kōta is her child. He is often comically presented as Beel's rival, constantly outdoing the future demon king in anything he attempts, which usually results in Beel crying and shocking Oga, who is usually unaware of what has transpired.

Relationships
Aoi Kunieda: Aoi clearly loves her baby brother, as she uses her alter ego in order to distance him from her more well-known identity as the leader of Red Tail, which could potentially make him a target for her enemies. She is usually the one seen taking care of Kōta, and is the relative that Kōta responds most favorably toward so far.

Beelzebub IV: Though they are rivals who are constantly trying to outdo each other, they are still friendly at times, as neither appears to have any other friends their age.

Hajime Kanzaki: Kouta assisted Kanzaki when Kanzaki was searching for a solution to his new technique. Kanzaki eventually found the solution after he saw Kouta rolling down a hill.

Trivia
In the Beelzebub anime, several filler episodes are often between plot developements. In one such, Kota is shown at a day care playing, with Beelzebub and his friend the cat accompanying him. Kota is here left alone, unable to play with the other boys, but Beel's appearance makes him happy, and the two friends together beat the other babies in a game of who could stack the highest blocks.

The anime presents them as two regular friends, and this is likely due to the fact that the manga chapter depicting them as rivals had not yet been released (in the manga, after their meeting at the park Beel is not shown with Kouta for another 80 chapters or so). As such, the relationship they have is quite different for each media.