A person who seems like a nice, quiet person, but is really a troll when online. They act sweet and innocent in person, but their secret hobby is trolling.
Hey man, did you know Suzy was such a closet troll? Yeah, she seems so nice and quiet at school, but she totally trolls online.
Wow, you're so nice around people, but I've seen some of your comments. You're such a closet troll!