A 20 year old NEET and high school graduate. His nickname is "Paisen". He is a cheerful man who speaks in a Kansai dialect and is very rich.
His mother died when he was very young, and his father, whom he has never met, wires him large sums of money to live off of. The name "Hidero" was given to him by his father, because his face was so ugly that he called out "hide yarou!", meaning ugly bastard.
When he was still in school, everyone in his class hated him, so he was not allowed to participate in cultural festivals, and was locked in the bathroom all day. He comes to his old high school every day to hang out with Tobio and his friends, who are his juniors in the futsal club, because he wants to have friends.
In the Yaba High School bombing case, a bomb he haphazardly placed near a propane gas pipe caused a catastrophe and killed 10 students. He was caught on a security camera and attempted to flee the country, but was arrested at the airport. He was released after his father, Sojuro Wajima, frames Mikio Maranaka, a homeless man, as the culprit, but the his lawyer tells him the truth about the incident, and he jokingly reveals it to Tobio and the others when they meet again. He jokingly reveals the truth to Tobio and the others when they see him again. Detective Iimuro also tells him that Wajima is his father, and that Wajima covered up the incident not to protect Paisen, but to avoid becoming a father of murdererer, and that he does not love Paisen. He meets his father for the first time in more than ten years, but he finds out that he knows nothing about the bombing, that he was not involved in covering up the incident, and that his very existence has been forgotten. He finds out that his father never loved him. So, to get his attention, he causes a giant uproar in Shibuya and confesses his crimes to the public. He is kidnapped by Reimu, his half-brother, and his friends, who have been ordered by his father to kill him and his friends. Killing Reimu in retaliation, the police arrested him and he was sentenced to 10 years in prison for murder. The event was changed by Wajima to "a riot by Kosaka Hideo, who thought he was the real culprit and blackmailed three students into joining him," and his blood relationship was erased. Ten years later, he is released from prison and reunited with Tobio and the others. When Tobio, who saw Reimu get stabbed to death ten years ago, asks Kosaka about his dream to become a comedian, he accuses him, "What kind of murderer talks about his dream so happily? But he replies, "I'm alive, so I can't help it, can I? When Tobio says that he sometimes feels like dying, he advises him that "feeling like dying is proof that you are alive.

(Source: Wiki)

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