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.