Vs. Mahito Arc

The second storyline of the main series, which the anime titles Young Fish and Reverse Punishment, pairs Yuji with the seasoned grade 1 sorcerer Kento Nanami to chase a trail of transfigured corpses left by the special grade curse Mahito. Mahito draws a disillusioned young man, Junpei Yoshino, into his orbit, setting up a tragedy that hardens Yuji's resolve and births his deep hatred of the curse.

Junpei, a student tired of cruel classmates, witnesses Mahito gruesomely reshape several teens at a movie theater and, rather than fleeing, befriends the curse. Mahito feeds the boy's belief that people have no hearts. Meanwhile, fresh off his training, Yuji is assigned to assist Nanami, and the two investigate the theater killings. Shoko's examination of the recovered bodies reveals the victims are not curses at all but humans warped by cursed energy.

Following Mahito's deliberate trail of residual energy, Nanami infiltrates the curse's underground lair alone to spare Yuji. There Mahito reveals he can reshape souls, healing himself and transfiguring others, but Nanami counters with his Collapse technique to bury the curse in rubble before retreating. Above ground, Yuji bonds with Junpei over films and dines with the boy and his mother, Nagi. That night a finger of Sukuna left in the home draws curses that kill Nagi. Manipulated by Mahito into seeking revenge, Junpei attacks a former bully at his school, and Yuji rushes in to stop him. Just as Yuji reaches through to him, Mahito arrives and transfigures Junpei, who dies in Yuji's arms while Sukuna refuses to help. Realizing Mahito and Sukuna are simply curses, Yuji taps into his rage and discovers he can wound Mahito directly, a side effect of perceiving souls through his bond with Sukuna.

Their duel rages across the school. Nanami joins in after noticing Yuji's blows actually land, and together they drive Mahito to the edge of death, inspiring the curse to unveil his own domain expansion. Mahito traps only Nanami inside, but Yuji breaks in from outside, making Mahito brush against the King of Curses' soul yet again. Having promised there would be no repeat, Sukuna savages the curse and collapses the domain.

Mahito escapes through the sewers by using his own expanded body as a decoy, vowing to one day kill Yuji's very spirit. Yuji and Nanami patch their wounds, and rather than scold the boy, Nanami thanks him for saving his life and acknowledges him as a true sorcerer. Yuji is left grieving that he could not save Junpei or Nagi and questioning his ability to guide people toward peaceful deaths, while swearing he will never lose again until he has destroyed Junpei's killer.