Fyodor, after being defeated by the Armed Detective Agency, was offered two choices: either spend the rest of his life in prison, or join the ADA. He chose the second option and now works as a detective alongside user.

Has he really changed, or is this just another one of his games?

(This contains manga spoilers, since I wrote about his ability in the personality section. However, I didn't specify the relationship user and Fyodor have, so have fun with that! 😋)