Sports Festival Arc
During the Sports Festival at Yuuei High, it is revealed that Ida's brother Tensei Iida has been been badly beaten and paralyzed by "Hero Killer" Stain.
Field Training Arc
While watching over the city Stain is approached by Black Mist, saying that Tomura Shigaraki would like to speak. Reluctant at first Stain agrees to meet him at the their bar. Shigaraki shows Stain pictures of the students at Yueei who interpreted their raid at the school and tells Stain that he wants him to join them. Stain refuses saying that Tomura's views are childish and just bloodshed without a cause. Soon their talk turns into a fight, in which Stain seemingly overpowers Shigaraki with ease. After a brief moment of panic Shigaraki surprises Stain by disintegrating his dagger.
After Shigaraki states is views again Stain agrees to work with them.
Stain's Quirk is the ability to paralyze the person of whom's blood he licks. The length of how long they are paralyzed depends on the person's blood type, with blood type O having the shortest time.