Mon-Chan was a child, one of the most loving boys Yukiji had ever known, along with Yoshitsune. He loved to play with Kenji and his friends, but he seemed closer to Konchi and Caw-Caw, as he was often seen with them more than with Kenji Otcho, Yoshitsune, and Maruo in flashbacks. Despite this, he was still part of their group.
As a young adult, after moving to Germany, Mon-Chan became deeply involved in rugby, a sport in which he excelled. However, when his mother fell ill, he stopped playing rugby to care for her and focus on his work. Although he did well in his career, his mother passed away eight years later. Following her death, Mon-Chan decided to start his own business with a colleague.
In 1997, while his business was thriving in Germany, Mon-Chan occasionally returned to Japan, where he reconnected with his childhood friends. In 2000, he immediately responded to Kenji's call after learning that Kenji was suffering from an incurable disease. On December 31, 2000, Mon-Chan, like the others, was terrified but managed to survive the ordeal.
In 2002, Mon-Chan was found bedridden in a hospital, aware that his illness had reached a critical stage. Yukiji visited him regularly, and Mon-Chan showed her some findings from his investigation into Friend's organization: excerpts from the "new notebook predictions."
Shortly after this visit, Mon-Chan left the hospital without informing Yukiji. He tracked down Sadakiyo to question him about Friend, his plans, and his accomplices. This encounter allowed him to reminisce about his childhood and have a heartfelt conversation with Sadakiyo. However, while believing that Sadakiyo lived day by day, Mon-Chan was killed by his "old friend," who took the memo containing all the information Mon-Chan had gathered. Yoshitsune and Kanna learned of Mon-Chan's death in 2014, although they had long suspected that his illness would eventually claim his life.
