WARNING: The following contains spoilers for My Hero Academia Chapter 336, "Villain” by Kohei Horikoshi, Caleb Cook, and John Hunt, available now in English from Viz Media.
In a bait and switch move My Hero Academia is now getting notorious for, the identity of the true UA traitor was disclosed in the latest chapter. A similar reveal happened in Chapter 335, when it was heavily hinted that Toru Hagakure was All For One's mole at UA. It turned out to be a set-up for Toru to discover the actual UA traitor: Yuga Aoyama.
Yuga has always been one of Class 1-A's most enigmatic members, but he was always known for being jovial and good-natured even in the face of undesirable circumstances. That's why it was easy for Toru, who was often paired up with him, to notice the sudden shift in his personality after the Paranormal Liberation War. Toru also noticed he hadn't smiled since 1-A had returned to UA with Izuku Midoriya. Yuga's depressive state bothered Toru and as he headed out unto the UA grounds, she shadowed him to see if she could offer some comfort. Instead, she discovered a startling secret.
Yuga was having a conversation with his parents in secret. Unbeknownst to them, Toru could hear every word and she learned exactly how Yuga had fallen into All For One's web. It turns out Izuku was not the only member of Class 1-A who was originally Quirkless. Yuga didn't have one either at birth. Like Izuku, he would have been bullied and ostracized for not possessing one and his parents hated the idea. As an attempt to ensure he lived a normal life, they sought a Quirk for him from All For One.
The idea of a previously Quirkless Yuga has been hinted at previously in the series. He mentioned that his Quirk was unsuited for his body and that his belt support item had been with him since infancy. His Navel Laser also used to give him a stomachache when he shot for more than a second, but he seems to have overcome this particular weakness. Just like Izuku, Yuga has worked to make the Quirk he received his own.
Since All For One granted Yuga a Quirk, his parents have been under the villain's influence. The contractual nature of All For One's "gift" indebted them to him and he expected Yuga to return the favor as soon as he was able to. Under the threat of losing his parents, Yuga was forced to pass information about his classmates' whereabouts to All For One. He was indirectly responsible for both the USJ incident and the training camp invasion.
Of course, Yuga isn't corresponding with All For One because he wants to. To him, this is the only way to protect his parents. His role as a spy isn't one he's comfortable with either and when Toru brought Izuku to confront him, Yuga broke down crying. Izuku finding out might be for the best, however. Currently, he's powerful enough to protect Yuga and his parents from a physical attack by All For One.
Unless the Aoyama family has inbuilt booby traps similar to the one that took out Lady Nagant, they're safe for now. They also have the unique opportunity to feed All For One false information and lead him into a trap by the UA students and Pro Heroes. Whatever happens, it's almost certain that Izuku will prioritize his friends' protection, and Yuga might be able to redeem himself yet.
Though Yuga being revealed as the traitor comes as a surprise, it does explain a rather quirky habit he has. In almost every frame where he's present, Yuga is staring straight at the audience rather than engaging with his classmates. Looking back with hindsight, this is an unnervingly obvious tell that he's always been watching.