The existence of a traitor amidst My Hero Academia's U.A. High School has been introduced and teased for a while now, with subtle clues being sprinkled throughout the first three seasons. However, while fans have long since speculated who the traitor might be, further development in this subplot has been dormant for quite some time. Plus, with the manga entering its final arc, fans wonder if there is even enough time to fully explore the traitor's reveal.
Let's discuss if this subplot has potentially been abandoned or if series creator Kohei Horikoshi may have another significant twist in mind.
Present Mic first brought up the idea of a traitor following Katsuki Bakugo's capture during the Forest Training arc. The events surrounding Katsuki's capture were deemed suspicious since the details and placement of this training camp were kept under strict lockdown. The only ones to know were the Wild, Wild Pussycats, U.A.'s faculty and the students in classes 1-A and 1-B.
Such precautions were put in place after the USJ incident, where the League of Villains seamlessly infiltrated, intending to defeat All Might. This foray required insider knowledge of All Might's teaching at U.A. and the school's private schedule. But somehow, the League of Villains gained access to this information, shocking U.A. and planting seeds of doubt in heroes among society.
In addition to these major incidents, other minor occasions suggest the work of a traitor leaking insider information too. For example, Shigaraki knew when Class 1-A -- Izuku Midoriya in particular -- was going to be at the mall and used that opportunity to question Izuku about the Hero Killer: Stain. Furthermore, the villains knew about the tracking device that Momo Yaoyorozu placed on the Chainsaw Nomu and planned accordingly.
Principal Nezu does suspect the students and his colleagues, but despite his intelligence-based Quirk, he has yet to snuff them out. One of his tactics involved implementing a dormitory system for both students and teachers. To the public, the system is meant to ensure the safety of the students, and while that is true, Nezu has an additional motive -- wanting to keep the students under close observation.
A possible traitor among the heroes-in-training has sparked several theories, many of which are plausible, as fans have spent years examining the smallest details to find more clues. From Neito Monoma to Yuga Aoyama, every character has been suspected at one point. However, there hasn't been any substantial evidence of there being a traitor in a long time. One reason may be that Horikoshi merely forgot about the subplot, too busy focusing on other aspects of the story -- or perhaps there wasn't a traitor to begin with.
Still, it's hard to ignore the question of how the League of Villains managed to obtain such sensitive information on multiple accounts. Taking a closer look at past incidents, the traitor hasn't made a move since All Might's notorious battle against All for One in Kamino Ward, which resulted in All for One's capture. This could imply that the traitor is loyal to All for One, exclusively, and not the League of Villains as a whole. If this is true, the traitor's silence makes a lot of sense. But with All for One's recent return in the manga, this could lead to the reveal fans have been waiting for in a significant twist.
While the current trajectory of MHA's story doesn't appear to leave much room to delve into a traitor reveal and backstory, it is unknown how long the final arc will be. There is still much to tell in Izuku's story, and a traitor among the U.A. might be a part of that. Horikoshi may have the traitor lurking in the shadows, waiting to reunite with All for One as an ultimate reveal during the series final arc, perfectly tieing back in a seemingly forgotten subplot.