Possession

I've had this idea for a while now. So, in media, when demons possess people, they're able to use their abilities, at least to some extent. I was thinking that since Grim is pretty capable and controls his own demon, Koza (the skull-headed demon I draw with him sometimes), he would allow Koza to possess him. But instead of fighting each other for control, they would work together and combine their magical abilities. This is why Grim has Koza's orange magic mixed with his own, some orange in his eyes, Koza's demon wing things, and the weird effect I give Koza's incorporeal body. Grim is mainly in control, but Koza helps out as well, and can provide a sort of warning sense (kind of like a spidey-sense but not exactly).  This way, Grim can handle stronger demons and other threats both supernatural or normal. 

Koza is technically stronger than Grim magically, but Grim doesn't tap into as much power as he has, since he's afraid to do so. He only does that when he's experiencing intense emotions, like fear or anger. While possessing Grim, Koza can push him to use more of his magical abilities.

TL;DR: Grim lets Koza possess him and they work together to be stronger.

Sorry for the long description. I probably left a lot of stuff out and it probably doesn't make much sense. I have a whole world put together and some rules for demons, summoning, magic, and stuff like that, but whatever. It makes sense to me and right now that's what matters.