Yashiro's Brother

Yashiro's Brother is a posthumous Antagonist but has some important story in Yashiro's part, he only appears in the manga.

Biography
Yashiro's brother was abusive towards his brother to the point that he forcibly gets Yashiro to keep guard as he lured a young girl into the shed, and raped her. Yashiro would then manipulate the girl into not telling the truth of what happened to her. Since then, Yashiro's brother would rape a young girl monthly, and Yashiro took a thrill out of manipulating them into getting molested by his older brother. However, when Yashiro wasn't keeping watch over the shed, their mother began to approach the shed; out of desperation, his brother attempted to silence his recent victim, but he accidentally ends up killing her.

Fearing the consequences, Yashiro's brother attempts to pin the murder on Yashiro. Realizing his brother's intentions, Yashiro murders him, and hangs him up with a noose in order to give off the illusion that he had killed himself. Despite being a one-time character, his brother left a lasting effect on his mind. Eventually, Yashiro attempted to lure children to their deaths, but he wasn't successful. When he was to marry a child psychologist, he took advantage of her knowledge on child psychology, and used that to commit several successful murders.

In the final chapters of the manga after Yashiro was arrested, Yashiro's Brother was mentioned by Yashiro as in he admitted the murder of his older brother, finally allowing him to rest in peace despite he was a villain.

Trivia

 * He is the only villain to be absent from the anime adaptation of the manga, it's most likely due to the graphic content pertaining to Gaku's back story. He also didn't appear in the live action adaptations (both 2016 and Netflix version).
 * He is also the only ERASED villain to die (since both Akemi and Gaku were brought to justice).
 * It is never explained why he was abusive towards Gaku and how long he was abusing him.