noun
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.
彼はアイドルグループの隠れファンで、誰にも言っていない。
He's a closet fan of the idol group and hasn't told anyone.
実は私、あのアニメの隠れファンなんです。
Actually, I'm a secret fan of that anime.