Pet.

There are people who can get into people's minds and control their memories. This power is used to eradicate mysteries and cases or worse, to assassinate. The potential of said power is strong enough that it is able to destroy people's minds, however, it can backfire and eat up ones own heart. To counter it, chains are used to lock and protect each other's weak and dangerous hearts. From one's growing fear and disdain, they're consequently called as a "Pet."