he's been around since the roman empire and has a long standing history with demons to the point that many of them have sought out to enslave/control him. he's a slippery individual though and has managed to elude that kind of dynamic in favor of an independent existence. maybe a demon that once sought to control him or someone he spurned, i love cat & mouse things like that. something simpler like a confidant or someone he's worked for before on various endeavors could be fun too.