Ryusui Seiryoin (born 1974) is a Japanese novelist and translator from Hyogo Prefecture. While studying at Kyoto University, he received the second Mephisto Prize for his 1996 debut novel, “Cosmic: Seikimatsu Tantei Shinwa.” He developed the JDC, or Japan Detectives Club, series through stories that combine large casts of detectives, verbal puzzles, symbols, and mysteries of extraordinary scale. His work has also encompassed manga scripts, business books, and English-learning guides. As editor-in-chief of The BBB, he has helped translate and introduce fiction by Japanese writers to international readers. His representative work, “Cosmic: Seikimatsu Tantei Shinwa,” begins with a declaration that 1,200 people will be killed in 1,200 locked rooms and follows the detectives of the JDC as they confront the resulting series of impossible crimes.