Aight! Here are some notes:

I'm thinking that the demon is either a lower or just barely a medium rank. That way, they can still be about the same level as the other characters.

See, while the demon isn't nearly as powerful as higher ranked demons, they make up for it with creativity, skill, and wit. I'm thinking enhanced agility and speed, illusions, and even a bit of hypnosis for them.

Also, I'm thinking of the human's appearance changing when the demon takes over. Like, not much, just their pupils narrowing a bit and their eyes getting a bit of a reddish hue.

As for the demon's personality, they're probably petty, childish(but surprisingly also very intelligent), and possessive of their human, while the human is more serious and straight laced. The human could even be a Straight Man if the other characters are crazy, hehe-

The two of them bicker [i]a lot,[/i] but at the end of the day, they truly do care for each other, so much so that the demon doesn't want to leave the human.

Oh, and also, maybe the human has earth/nature magic as well?

Comments, questions, and suggestions are encouraged! <3