Who is Ryoichi Gotanda from Dance Dance Dance: A Novel and what is their importance?

Ryoichi Gotanda is a former junior high school classmate and former science lab partner to the writer. When Kiki disappears, the writer contacts Gotanda hoping that he can provide him with information. Although they hadn't spoken in over twenty years, the two men begin to spend time together, drinking, eating, and attempting to unravel the mystery of Kiki who seems to have disappeared without a trace.