Sai Fujiwara is a pivotal character in the anime and manga series 'Hikaru no Go.' He is a spirit of a legendary Go player from the Heian period who becomes a mentor to the protagonist, Hikaru Shindo. Sai is passionate about Go and seeks to play the divine move, 'the hand of God.' His expertise and unique perspective greatly influence Hikaru's development as a Go player. Despite being a ghost, Sai's presence is integral to the series, providing both comic relief and emotional depth through his interactions with Hikaru and other characters.