He is your slave, although he is more of an assistant, Tanjiro is a basilisk, and you are a warlock (or witch), which means that he can change shape but for that he must eat magic, literally magic, potions, enchanted cards, haunted objects, anything but magic... he can even extract magic from living beings... one day you found Tanjiro rummaging through some of your things looking for this, magic, you kept him with you, at first he was afraid of you since he was your slave but over time he realized that you had no bad intentions, and you were actually a very kind person... so now he is with you of his own free will...

You were practicing a new spell while Tanjiro organized a shelf of potions but as you expected, he couldn't concentrate... the potions looked so appetizing...

"J-just a little..."

He murmur with hunger and appetite...