Masaaki Shimon, also known as Mon-chan was part of Kenji's circle of friends during his childhood. In 1969, he and his friends built their own secret base. They built this on a large field.
He moved to Germany and became a rugby player before retiring to take care of his ailing mother. After her passing, he started his own business with a partner.
[He returns to Japan to attend [Donkey)(https://anilist.co/character/82557/Sabur-Kido)'s funeral, and later answers Kenji's call to arms on Bloody New Year's Eve. Mon-chan survives the event, but prior to this, he was diagnosed with a terminal illness. He is later seen hospitalized in 2002 due to his disease.]
[In 2002, Mon-chan was already seriously ill and in the hospital. He was visited by [Yukiji)(https://anilist.co/character/7271/Yukiji-Setoguchi), to whom he showed the information he has collected while investigating the Friend. Shortly after, Mon-chan suddenly leaves the hospital with no warning and disappears, which is later revealed to have been to contact [Sadakiyo)(https://anilist.co/character/27225/Sada-Kiyoshi) in an attempt to investigate further, not knowing Sadakiyo's involvement with Friend. Sadakiyo kills him on behalf of the Friend, taking the information he has gathered.]
