Shimon Masaaki, also known as Mon-chan, loved to play with Kenji and his friends. It seems that he was closer to Konchi and Caw-Caw since he was seen more often with than Kenji, Otcho, Yoshitsune, and Maruo in flashbacks.
After moving to Germany, he became involved in rugby, a sport in which he excels. Then, his mother fell sick which led him to the decision of stopping his rugby career. He cares for his mother and is dedicated to his work. He is doing very well, but his mother died eight years later. Mon-Chan decided to start his own business with a colleague.
In 1997, while his business is booming in Germany, Mon-Chan returns from time to time to Japan where he meets his childhood friends. In 2000, he immediately responds to the call of Kenji. Before that, he found out that he was suffering from an incurable disease. On December 31, 2000, in the evening, he was just as terrified as the others. Still, he manages to survive.
In 2002, he is nailed to a hospital bed. He knows that his illness has reached a critical stage. Yukiji checks up on him regularly. Mon-Chan reveals to her what he found while investigating. Such as the organization of Friend, excerpts from the "New Book of Prophecy", predictions, etc. But shortly after this, Mon-Chan left the hospital without informing Yukiji. He manages to find Sadakiyo. He asked the identity of Friend, his plans and his accomplices. This is an opportunity for him to remember his childhood and have a chat with Sadakiyo.
Mon-Chan is killed by his "old friend" who gets his memo, containing all the information he recorded before. The killer tries to convince himself that he is a "good". Yoshitsune and Kanna learn about his in 2014, even if they suspected that his illness would get to him one day or another.