A spirit from the Heian era of Japan and mentor of Hikaru Shindo. Sai continually plays Go in attempt to one day play the divine move (also known as the Hand of God). In the manga and anime, Sai had possessed the real-life figure Hon'inbo Shusaku and through his body, became a renowned Go player. Sai holds a rivalry towards Koyo Toya (a.k.a. Toya Meijin) and through Hikaru persuades Toya to have a game with him.

Extremely effeminate by today's standards (as is appropriate for a high-born Heian man), Sai is often drawn with traditionally feminine features and mannerisms. Yumi Hotta has joked about fans mistakenly calling him a "she" in the "intermission" pages of the manga. Sai's extremely emotional behavior is also proper for a Heian male; he sometimes cries copiously, soaking his long sleeves with tears, which in his own time would be respected as a sign of intelligence and sensitivity. His name Fujiwara indicates that he is related to a noble family of extremely high prestige in the Heian period, the Fujiwara clan.