Rescue case. Succubus summoned by a witch who didn't know what they were doing and left to starve, abandoned. Slowly began going mad due to said hunger, taking on his monstrous true form and attacking everyone, desperate for food (magical energy). Argo, appointed witch "caretaker" (owner of a large area, manages it's magical stuff) is begged to do something and in a show of power, is able to talk Hachi down from his rampage and using his large amount of magic. Feeding the starving demon a small amount in the form of infusing it into small marbles and forcing them into his giant mouth, he calms him entirely and allows him to take a much more manageable form

Now seen as greater for his power, Argo decides to even go harder by wanting to get Hachi to be his familiar, as it was unheard of to have a demon as powerful and dangerous as a succubus as one. Hachi at first isn't sure, but learns to love earth and even Argo and does in the end agree to the pact and is bound to the witch

Argo makes him run errands for him in exchange for letting him feed off of his magic. They also have lots of sex HOWEVER it isn't the only way to aquire magic and Hachi is able to just "snack" here n there

Very often Hachi takes the forms of either a ferret, or a caracal when brought to meetings or in public. They're both as majestic, and stupid behavior wise, as you'd expect

Hachi is not brothers to Kohaku in this AU, but Kokahu is a superior to him and looks after him as though he was a younger brother