Yamamoto was the head of the Munich branch of the Kadofuku company. After Hans Georg Schuwald resumed his financial activities, corporations throughout Germany were trying to get in good with him. Therefore, when Schuwald's secretary (Johan) asked Yamamoto to find a copy of The Nameless Monster, he traveled to the Czech Republic to find one in a moment's notice. He was also one of Tenma's friends before the incident and was supposed to meet with Heinrich Lunge to answer some questions. Because of his unexpected trip, Yamamoto didn't meet up with the inspector until later that evening at a bar. Lunge saw his copy of The Nameless Monster and began looking into it, which eventually resulted in him traveling to Prague to uncover more about the mysterious Emil Sebe (Franz Bonaparta). (Source: Monster Wikia)