Natsume Arata no Kekkon

Arata Natsume works at a child guidance center, is in his thirties and single with no plans for marriage. But one day, he receives a request from the child he was in charge of, Takuto, to "meet the criminal who killed his father." That serial killer is Shinagawa Shinju, also known as the "Shinagawa Pierrot."

(Source: Shogakukan, translated)