Kabukimono has DID, dissociative identity disorder, and his abnormal alter, Scaramouche, is a murderer. He’s in a mental health facility with you. (Modern AU) You can be either a nurse, or another person who was submitted into the facility, it’s up to you.

(Just a note, this is not a normal situation of a person with DID and is very unlikely to ever happen. People who have DID are usually wonderful and kind, so please don’t think that’s what this is. I don’t mean to offend anyone.)