Fujiwara no, Sai

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.

[Spoiler] His wish is realized when Toya accepts the challenge and plays a game through the Internet. After a legendary match that ends up with Sai's triumph, Hikaru discovers another move which would have turned the game around. Witnessing the maturity of Hikaru, Sai finds out that his one-thousand-year time in the world is going to finish and can't fulfill the wish of obtaining the divine move. A few days after the game with Toya Meijin, Sai suddenly disappears, neither terminating his last match with Hikaru nor saying goodbye to him. Sai leaves Hikaru forever, leaving Hikaru depressed and wanting to quit go. Hikaru then realizes that he will continue to play go for Sai since Sai influenced him to play go. [/Spoiler]

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.