secret admirer; closet fan
Refers to someone who is a fan of a person, group, or thing but keeps it hidden, often out of embarrassment or social pressure.
He's a closet fan of the idol group and hasn't told anyone.
Actually, I'm a secret fan of that anime.
Compound of 隠れ (hidden) + ファン (fan). The exact origin is uncertain, but it likely emerged in modern Japanese to describe fans who conceal their enthusiasm.