The vice-captain of Shitenhoji, but has almost no role in the story. He was originally intended to play a larger role, but this was cut when Konomi had to move Chitose to Shitenhōji, and the team wound up with an extra player. Koishikawa is more known for analyzing the situation and the atmosphere of the matches than for his capability to play. He depends on his calm judgment to support Captain Shiraishi from the shadows