Warning: contains spoilers for My Hero Academia chapter 320!
One of My Hero Academia's first big villains, Hero Killer Stain, may turn out to be essential to setting Deku back on the right path and saving the day, acting as the final mentor in his quest to become a real hero. Stain has propelled the series' plot by drawing attention to the League of Villains and offering sincere criticisms of the hero society, and despite his early defeat, he's remained an important influence. Now, as Deku operates alone as a vigilante, the time has come for this figure to make a comeback.
In his first appearances, Stain attacked heroes who he felt weren't living up to the ideals of what a hero should be, such as those expecting compensation or fame and adoration. In Stain's view, the only one truly worthy of being called a hero was All Might, whose utterly self-sacrificing behavior met his high standards. Stain had previously tried a vigilante career as Stendhal, where he killed criminals guilty of even minor infractions; it was only after having his vigilante actions interrupted by pro heroes that he began to see false heroes and villains as being the same. Stain very nearly killed Deku, but ultimately spared him because he saw the same characteristics as All Might in the boy.
When All For One broke free from prison, Stain was able to escape as well, alongside many other villains. While he's kept a low profile, it's become apparent in recent chapters that he's been following All Might and Deku, although they don't seem to have noticed this yet. Since it's well known that these two are the only heroes to have earned his respect, it seems unlikely that he's simply planning to jump out and attack them. On the other hand, Deku's insistence on facing everything alone has caused him to undergo a dark transformation compared to who he was the last time he and Stain met. With his scarier costume and array of intimidating powers, Deku's appearance has been mistaken for that of a villain on several occasions now. Stain is observing for a reason, and My Hero Academia may be setting him up as the final arbiter of the kind of hero Deku has become.
Fortunately, most of Deku's behavior has still been aligned to his original beliefs, protecting the pathetic villain Chisaki and trying to save Lady Nagant and bring her back around to the good side. Stain might even approve of Midoriya's conflict with his fellow students, although that's far from certain. Stain may also be literally watching Deku's back, intending to help his heroes and fulfill his original intent of stopping those who abuse their power by saving him at a crucial moment. Deku has rejected his friend and allies so far, brushing away All Might and demanding Class 1-A leave him alone. It's looking more and more like the only person who could bring him to his senses and actually get through to him about the pitfalls of his new attitude is Stain. That's not to say that the Hero Killer is going to calmly talk Deku round - it's just as likely that his congratulations will be what make the young hero see he's lost his way.
Stain has always been one for big speeches, so no doubt whenever he reveals himself, he'll have a few choice words for Deku. While what they'll be is still a mystery, it's clear that Stain still has one more big role to play before My Hero Academia's story comes to its end. Whether it's returning Midoriya to his senses, stepping in to save his life from All for One's assassins, or a simple desire for vengeance against the League, Hero Killer Stain is shaping up to be the one who provides the young hero with one of his final lessons - a genius twist which would turn this longtime enemy into a central figure in Deku's final ascension to true heroism in My Hero Academia.