I can suggest you "Lil Abaddon" idea. I know you have a "Charge System" (How i call this fillng with green black ring), so you can give him this system. You need to charge him to make his ring 3x3 with him in centre when you need it (I need it sometimes). He'll cost 50 soul hearts. But maybe... It's already in the game, i don't know, i didn't find him when i played, sorry.
P.S. About Soul Hearts read in my post below