Masaaki Shimon, known to his friends as Mon-chan, is a childhood companion of Kenji Endo and a member of the group that reunites to confront the threat of Friend. During his youth in 1969, he was part of the circle that built the secret base and participated in the creation of the Book of Prophecy, though he was not among the innermost core of the group. He grew up to move to Germany, where he became a skilled rugby player. His athletic career ended when his mother fell ill, leading him to set aside sports to care for her and devote himself to his work. After his mother passed away eight years later, he started his own business with a colleague, which grew into a successful enterprise. Despite his life abroad, he maintained his ties to his old friends, returning to Japan periodically. In 2000, when Kenji called upon his former friends to stop Friend's plan, Mon-chan responded immediately. At that time, he had already learned that he was suffering from an incurable illness. On the night of December 31, 2000, during the Bloody New Year's Eve event, he was terrified but managed to survive. By 2002, his disease had reached a critical stage, confining him to a hospital bed. From there, he continued to investigate Friend's organization and discovered excerpts from the new notebook predictions. He then left the hospital without informing anyone and tracked down Sadakiyo, seeking to learn more about Friend's plans and accomplices. During this encounter, he reflected on their shared childhood. Sadakiyo killed him and took the memo that contained all the information Mon-chan had recorded. His death was discovered by Yoshitsune and Kanna in 2014, though they had already suspected that his illness would eventually claim his life. Notable abilities include his physical prowess as a former rugby player and his resourcefulness in gathering intelligence even while terminally ill. His primary motivation was a sense of loyalty and duty to his childhood friends, spurring him to act against Friend despite his failing health. Key relationships include his enduring bond with Kenji and the rest of the group, his fondness for Yukiji, and his grim final encounter with Sadakiyo, which ended in his death. His development traces a path from an active, successful life abroad to a tragic, self-sacrificing end driven by friendship and determination.