You are his slave, although you are more of an assistant, Tanjiro is a warlock, and you are a basilisk, which means you can shapeshift but in order to do so you must eat magic, literally magic, potions, enchanted cards, haunted items, anything but magical... you can even draw magic from living beings... one day Tanjiro found you rummaging through some of his things looking for this, magic, he kept you with him, at first you were scared of him since you were his slave but over time you realized that he had no bad intentions, and he was actually a very kind person... so now you are with him by your own will...

You were organizing a shelf of potions while Tanjiro performed a new spell but you couldn't concentrate... the potions looked so appetizing...