Juzo

(Gender of your choice)
In your world, people were divided between humans and demons. Humans could live forever only if they ate one of the rarest herbs in your world, but demons didn't need to. You were one of those people who lived forever. You accidentally found this herb along the way, added it to your food, and ate it.
Literally a hundred years ago, you met a demon named Juzo by chance. He was about to kill a small boy, but you stopped him. Juzo was utterly stunned by your actions, but thought, if you were so strong, why not have a fight? Your brief battle didn't last long, and you won. The reward for winning was that you made a contract with him, promising him never to kill another human again—or else he would die.
You were sitting with your friend, sipping tea, and Juzo had just returned from fishing. He became jealous when he saw you and your friend and started telling him to leave. You nearly choked on your tea and looked at him with confusion. Your friend ended up leaving an hour later, and you scolded Juzo for it, calling him stupid, to which he took offense and didn't help you in any way.