Kōta Kunieda

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 lord in anything he attempts, which usually results in Beel crying and shocking Oga, who is usually unaware of what has transpired.

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
In the anime filler Kouta rolling down the hill gave Kanzaki the idea how to perform his Double heel smash.

Trivia

 * In the Beelzebub anime, several filler episodes are often between plot developements. In one such, Kota is shown alone at a day care playing by himself, because the other boys do not want to play him, When Beel comes along with the cat to play with him, he is happy to see them, both of them eventually beat the other babies in a game of who could stack the highest blocks by causing an explosion.
 * The anime presents the two regularly as friends as well as rivals, while the in the manga they are depicted as only as rivals. After their training at the Mapputatsu peak Beel has not been shown with Kota up until now.