Seiki (青喜) is the foster-son of Shinshi. He was raised alongside Shinshi's son Eishuku and regards both Eishuku and Eishuku's wife, Shuka, as an older brother and sister to him. During the reign of Shishou, he loyally serves as the manager of Eishuku's estate.
A bright and clever young man, Seiki became a sennin at the age of 19 to serve as Eishuku's undersecretary. His astute nature allows him to provide insights into the mysterious deaths of the Taishi Daishou and Taiho Junkou, as well the nature of the kashokada, which results in Shuka realizing who is responsible for Shishou's increasingly erratic nature.
Upon Shishou's death from abdicating his throne, Seiki and Shuka are comforted by Shinshi, who tells them that a kingdom whose foundation is based on blame will not endure. Shinshi asks that Seiki finally show his true capabilities and take responsibility rather than hiding by working under other people, as Shinshi suspects that Eishuku will not live much longer. In response, Seiki refers to her as Kouko, the golden mother-in-law.