A spirit from the Heian era of Japan and mentor of Hikaru Shindou. Once the Go instructor to the Emperor, Sai continually plays Go, his goal being to one day play the divine move. Sai's ghost had previously possessed the real-life figure, Honninbou Shuusaku, who became a renowned Go player. He is an extremely emotional person, who often acts childish and has great mood swings. While playing Go however, his behaviour changes completely as he controls himself and his emotions to play to the best of his ability. Sai has a high sense of honor, abhorring cheating and bullying.

Spoiler

He holds a rivalry towards Kouyou Touya, and through Hikaru persuades Kouyou to have a game with him; with his wish realized when they play a game through the Internet. After the legendary match that ends with Sai's triumph by the narrowest margin, Hikaru discovers another move which would have turned the game around. Witnessing Hikaru's maturity, Sai comes to feel that his one-thousand-year time in the world is going to end without his fulfilling his wish of obtaining the divine move. A few days after the game with Kouyou Touya, Sai suddenly disappears as he feared he would. Hikaru was dozing during their last game, so he could neither terminate his last match with Hikaru nor say goodbye to him. Sai is gone, leaving Hikaru depressed and wanting to quit the game. Later when Hikaru plays Isumi, he realizes that he wants to continue to play Go.