I'm legit surprised this hasn't been brought in as a topic. I've seen several videos on this subject and many have theorized about it. Personally I believe it is Toru Hagakure due to reasons stated by Joy Boy below. He does a lot of One Piece and Boku no Hero stuff, and I highly advise you check him out. Basically the video says it all, but even then there is still, more like how the nature of her quirk is unknown. Is she really who she says she is. Why is she always not included in pivotal scenes? We've never seen her family, she's never had any development other then a SINGLE addition to what her quirk can do and honestly, it's just a quick blinding flash...But simply letting my bias dominate this theory won't do. So I'll post some other possible theories on who the traitor might be as well. I won't post any ones that make no sense though. Sorry "Ochako is the Trator" supporters, your arguments make no freaking sense, and are outdated. There is also a theory that the principle is the traitor but I couldn't find a video on the subject.