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.
At first, it seemed that the cheerful invisible Toru Hagakure was the long-suspected U.A. traitor, but Present Mic's theory about a traitor actually involves the sparkling hero Yuga Aoyama, not Toru. Yuga confirms in Chapter 336 that he is U.A.'s traitor, but not by choice. He operates under extreme duress from the villainous All For One.
All For One seeks more than a vast collection of Quirks with his own Quirk. He aims to rule the entire world and reshape it in his image as the Symbol of Evil, and most likely, a meticulous planner like him has hundreds or even thousands of agents like Yuga scattered all over Japan, or possibly the whole world. It's unlikely Yuga is the only traitor.
The Mass Manipulation Of All For One
When Izuku met One For All's first wielder, Izuku not only learned the truth of the One For All Quirk but also its creator, the villainous All For One, who wields a Quirk of the same name. In the early days of Quirks, no one knew how to handle these strange supernatural gifts. During these chaotic times, All For One took advantage of the situation and used his own Quirk to rearrange other people's Quirks on request, as Izuku saw for himself. All For One did favors for people who didn't like their Quirks, but in exchange, those people owed All For One their loyalty and obedience, and All For One amassed thousands of devoted followers over the years. He presented himself as a benevolent and visionary savior of society.
These actions make it clear how All For One operates. He wants to use the world's #1 resource, Quirks, to convince everyone that he is destined to lead humanity into a better future of Quirks and peace. All For One knows how much people care about Quirks, whether for good or ill, and he ruthlessly used this leverage in the old days to become a savior. This is not too different from Destro's cause, claiming that all Quirks/meta abilities should be used freely. Now and forever, Quirks control society rather than the other way around, and that continues to the present day where Yuga Aoyama and his family are concerned.
All For One's Leverage Over The Aoyama Family
In the present era, society has long since gotten used to Quirks in terms of societal norms, the judicial system and more, which restricts All For One's ability to recruit followers via Quirk manipulation. However, this villain still has an angle, that being Quirkless people. In Izuku's day, fewer people than ever are born Quirkless, but even a few hundred Quirkless people is all it takes for All For One to make his move. Already, he made a deal with the Aoyama family to give the Navel Laser Quirk to their Quirkless son Yuga, and that means the Aoyamas owe All For One a big favor. All For One holds Yuga's parents hostage, and combined with what Yuga owes to AFO, that makes it easy for AFO to manipulate Yuga.
This is a tricky and tragic situation for the Aoyama family, but in theory, All For One has struck a similar deal with dozens or even hundreds of other families across Japan, since Quirkless people are rare but not unheard of. Such double agents, operating under duress, can sabotage hero schools, pro hero agencies, the police, Japan's judicial system and more to reshape society according to AFO's wishes, and Yuga may be just one of many chess pieces AFO has on the board.
There may be a similar traitor student at the Shiketsu school or Ketsubutsu as well. With Quirklessness being so undesirable and stigmatized, it's easy to imagine hundreds of desperate families making this "deal with the devil," thus helping to lay the groundwork for AFO's eventual triumph. All For One is certainly smart and patient enough to gather so many followers and juggle so many agents at once. Surely, AFO isn't betting everything on just one kid with a belly laser.