My Hero Academia has answered one of the lingering mysteries of the previous War Arc, which is how the sadistic firebug villain Dabi managed to learn the true identity and origin of No. 2 pro hero Hawks. It was Hawks who got early wind that the League of Villains had merged with the Meta Liberation Front to form a mega-terrorist group, the Paranormal Liberation Front; it was also Hawks who went undercover in the group, to give the heroes an edge. However, when the battle went down, Dabi was able to use the truth about Hawks to help shatter public confidence in pro heroes as a whole.
Warning: My Hero Academia Chapter 299 SPOILERS Follow!
Chapter 299 of the My Hero Academia manga picks up after the war in Jaku City is over. A seriously-injured Hawks is getting a ride from Best Jeanist, to track down the lead of who betrayed the heroes spy operation to the villains. Jeanist faked his own death as part of that operation, in order to give Hawks the evil cred he needed to convince the League of Villains of his allegiance. Dabi was no fool, either: the heroes had to actually induce a deathlike state for Jeanist, and allow the villains to keep his "corpse" prisoner, all for Dabi to believe Hawks' story. And even then, Hawks' ruse wasn't enough!
We learn in Chapter 299 that Dabi discovered Hawks' true identity - more importantly, Dabi learned that Hawks was the son of a ruthless criminal and murderer. Hawks' fugitive dad had been harbored by his mom, forcing her and Hawks (real name Keigo Takami) to live inside, and away from the pursuit of the police and pro heroes. When Hawks' mom, Tomie, tried to leave the toxic relationship, they ended up on the streets. Things were grim until little Keigo's unique quirk and heroic nature got him and Tomie into protective care from the Hero Commission, with new identities.
In discovering this secret, Dabi was able to send thugs to Tomie's new home and coerce her into giving up the details of Hawks' origin. In his Joker-like rise to infamy during the war, Dabi revealed Hawks' dirty family ties, as well as the reveal that he (Dabi) is Endeavor's son Toya Todoroki. That one-two punch has dirtied the images of the two top pro heroes and undermined the entire pro hero system society is based around.