Here's a weird interaction I encountered. I had both Toula and the Luckier Tooth, but used up all my Death Defiances (including both of them). When I got to Circe, I took the boon where she gives your familiar an extra use of their power, so I got a new Toula death defiance. I still had this one Toula life after I defeated Eris, but since my Luckier Tooth life was already used up, I switched it for something else at the cabinet. The game apparently just removed my leftmost unused life instead of specifically removing my Skelly life, because it took away the Toula Defiance I got from Circe.