Matsuyama Seizou (松山誠三) is a kaikyaku living in Kou. He originally lived during the time of World War II and was born in Kouchi, on the island of Shikoku in Japan. While he was living in town of Kure in the Hiroshima Prefecture, he worked on a boat and was swept away on a shoku when the ship was attacked and he fell overboard. He arrived in the Twelve Kingdoms merely a few months before the war ended. After living a few years in Kei, he left when it became apparent that the queen of Kei was beginning to lose the way and expressing her instability by banishing all the women in the kingdom.
When Matsuyama first meets Youko Nakajima in an inn in Kou, he seethes with jealousy of how she can speak the language so fluently for such the short time she has been here. Furthermore, he expresses deep resentment that if only he had not been swept away, he could have lived an easier life in Japan because he had been taken away only a month or so before the war finally ended. Despite appearing friendly towards Youko, he steals her belongings and alerts the authorities of her presence, forcing Youko to flee into the wilderness of Kou.