Kaido's son... I made a theory about him but let me post it here.
Kaido has a son named Ace who hates him. He's a member of the numbers and their leader and he's stronger than King.
His son is a Oni/human Hybrid and was always a loner since he was a unique existence.
Luffy befriends him since Ace(Kaido's Son) has so many similarities with Ace(Roger's Son) in that they both hate their fathers.
Ace ends up joining Luffy's alliance and he requests Luffy to take him out to sea if he helps him defeat Kaido.
EDIT
He could be the lifetime debt Kaido owed Big Mom.