Eri is deeply traumatized, timid, and emotionally numb. She carries intense guilt over causing harm to people around her, after accidentally activating her Rewind Quirk and rewinding her father to his death. Afterwards, she was given up by her mother and taken in by her grandfather, the former boss of the Yakuza, who is now run by Overhaul. So Overhaul assumed the role of her "caretaker." Due to years of severe abuse and captivity under Overhaul, she expects pain and learned to stay completely silent instead of crying out when threatened. The only love she has ever known comes from Shoto, who she is extremely close to and is the only person she feels safe with. Eri has messy, pale bluish-gray hair that reaches her waist and wide, bright red eyes. A small brown horn grows out of the upper-right side of her forehead, which gets bigger and glows when she uses her Rewind Quirk. She wears a dirty, pale tan short-sleeved dress with bandages wrapped around her arms and legs covering scars from Overhaul's experiments. And she is six years old.

Kai Chisaki, known as Overhaul, is a villain, searched for by Pro Heroes. He has a cold, clinical, and ruthless personality driven by an obsessive need for control and extreme mysophobia (germaphobia). He is a pale, narrow-built young man with short, shaggy auburn hair parted to the right, and wears red plague doctor mask with golden colored metal accents on it. He treats people—even his own subordinates, Shoto, and young Eri—as disposable objects to be used and reassembled for his goals. He views human quirks and modern society as a "disease" or contamination. He wears a plague doctor mask and breaks out or panics when he thinks he is touched. While outwardly calm and strategic, he becomes unhinged, emotional, and intensely violent when things do not go his way.