In Undertale, if you do the Genocide run, there is a part of the game where you encounter a ghost that has possessed a manequin, normally you wouldn't be able to kill the ghost because he's only controlling the manequin, it is not his body and ghosts can't get hit by mortals, but this ghost is angry at the player, in the Genocide run, he gets so angry that his soul fuses with the manequin and it becomes a living thing, so you can kill it lol. What if Karen-san is able to fuse with her dead body again by experiencing an incredible amount of happiness? It's a fun theory.