From the middle grade fantasy books, The Bone Grit Historeum
A Simp is like a shadow-self, invisible to everyone but its owner, who sees it in the reflected world. It is generally manageable, more a nuisance than anything. Many people have led normal, happy lives, keeping their Simp in check by removing any and all mirrors and reflective devices.
There have, however, been examples of Simps overtaking their owners and becoming very dangerous indeed. Almost like a monster version of them. When this happens there is little help for the owner, who becomes a slave to the Simp's will.
Unfortunately to kill the Simp would be to kill the owner, they are so enmeshed.