Stain, also known as the Hero Killer, is a major antagonist in Boku no Hero Academia. He is a vigilante-turned-villain who believes only selfless heroes deserve to be called true heroes, and he targets those he deems unworthy. His ideology heavily influences other villains and heroes alike, especially in how society views heroism. Stain serves as a key ideological threat rather than just a physical one, challenging the moral foundations of the hero society.