Fujiwara no Sai

Sai is expressive, sensitive and deeply attached to Go. He can sulk when Hikaru refuses to let him play, become ecstatic at the prospect of a strong opponent and cry openly when emotionally overwhelmed. His courtly appearance and mannerisms contrast with reactions that are often surprisingly childlike.

At the board, Sai becomes composed and intensely analytical. He respects strong play regardless of an opponent's age or reputation and has little tolerance for cheating. His desire to reach the Divine Move is not simply a wish to win; he believes there is a perfect level of Go that generations of players are collectively approaching.

Sai also grows genuinely attached to Hikaru. Although their relationship begins because Sai needs Hikaru's hands to play, he gradually becomes a teacher who takes pride in Hikaru developing a style and ambition of his own.

Sai is an extraordinary Go player with centuries of accumulated experience. His reading ability, positional judgment, endgame calculation and tactical awareness place him beyond virtually every player Hikaru initially encounters.

Before appearing to Hikaru, Sai's spirit had attached itself to Honinbou Shuusaku, the historical Go master whose recorded games Sai claims to have played through him. Those games preserve evidence of Sai's strength long after Shuusaku's lifetime.

Sai adapts readily to modern Go despite coming from the Heian period. Internet Go gives him an especially useful outlet because he can compete anonymously through Hikaru, attracting attention from strong players who know him only by the online name "sai."

Sai emerges from the bloodstained Go board found in Hikaru's grandfather's attic and persuades the initially uninterested boy to let him play. Their first visit to a Go salon brings them into contact with Akira Touya, whom Sai defeats through Hikaru's hands.

Hikaru's exposure to Sai gradually turns an arrangement made to satisfy a ghost into a genuine interest in Go. As Hikaru begins insisting on playing his own games, Sai shifts from player to teacher and must accept that his student's growing independence means fewer opportunities for Sai himself to sit at the board.

Sai eventually receives the game he has long wanted against Kouyou Touya through Internet Go and defeats him by half a point. Hikaru's later analysis reveals a move that could have changed the result, leading Sai to realize that his centuries of existence may have been intended to pass Go onward to Hikaru. Soon afterward, Sai disappears. Hikaru initially abandons Go in grief before recognizing during a game with Isumi that Sai's Go continues through his own play.

Hikaru Shindou is the only person who can normally see and hear Sai in the modern era. Sai begins by using Hikaru Shindou as his means of playing Go, but their relationship develops into a close friendship and mentorship.

Akira Touya becomes fascinated by the overwhelming strength displayed when Sai plays through Hikaru. Because Akira Touya cannot see Sai, he attributes that strength to Hikaru himself.

Kouyou Touya is the modern professional Sai most strongly wants to face. Sai eventually plays Kouyou Touya anonymously on the Internet and defeats him by half a point.

Honinbou Shuusaku was Sai's previous human host. Sai states that the games attributed to Honinbou Shuusaku were played with Sai's guidance.