Who is that former detective who disappeared?

"Speaking of which, have you asked your grandpa about that
former detective?"

"—I did."

"What was the reply?"

"He didn't tell me, told me to find out myself."

"—Is this also part of the detective’s training."

"Yes. But he did tell me one thing– the nickname of the former
detective."

"Nickname?"

"Yes, what the former detective is best at is disguise. He is so
deceptive that no one has ever seen his true face. I heard that
detectives call this kind of unidentified person– 'Variationist'