Seishu (清秀, Seishuu) is a young boy with failing eyesight who winds up travelling with Suzu. He lost both of his parents to youma. Seishuu originally came from Kei, but when Jokaku ordered the exile of all the women in the kingdom, his family decided to leave. His father was unfortunately attacked and killed by a youma, devoured by a demon right before Seishuu's eyes, while Seishuu was injured from a concussion to the back of his head. The injury gradually grows worse and is the source of seizures and his failing eyesight.
Seishuu and his mother eventually made their way to Sou and then Sai, but his mother passed away from illness before they can return to Kei. When the new ruler ascended the throne of Kei, Seishuu elects to return and encounters Suzu making her way to Kei by boat. He and Suzu initially do not get along, since he openly points out that Suzu, despite her difficult life, made no actual effort to try to change those circumstances and only moped about how unfortunate she was. However, they gradually grow on one another and become friends.
During their voyage, it becomes apparent that Seishuu's health is failing and Suzu becomes convinced that they must go to Gyouten and meet with the monarch of Kei, who Suzu believes can heal Seishuu. However, they are separated in Shisui Prefecture in Wa Province, where Seishuu is deliberately run over by the carriage of the prefecture's praetor, Shoukou, for collapsing in the carriage's path. Seishuu dies in the arms of a girl named Youshi, who passes on Seishuu's dying words to Suzu that he was hoping that she would not cry because of him.
Suzu realizes upon his death that all of Seishuu's words regarding her own self-pity were true and resolves to become stronger. Devastated by the death of her friend and angered that the ruler of Kei would allow such a person to govern others, she swears to avenge Seishuu by killing Shoukou and the ruler of Kei for her inaction. However, Suzu's anger is tempered by Sekki and Koshou, who recruit her to join their rebellion, and by the admission by Youshi, revealed to be the monarch of Kei, that she would have saved Seishuu if it had been in her power to do so.