Kiteretsu Sai, also called Kiteretsu Saisama, is the Edo-period ancestor of Eiichi Kite and creator of the original Kiteretsu Encyclopedia. An inventor branded a "troubled world with a shady operator" by authorities, he endured house arrest. Despite confinement, Sai concealed his inventions within the Encyclopedia, passing it to his descendants.

Eiichi Kite meets Sai during travels to the Edo era. On one journey, Sai repairs a malfunction in Eiichi's time machine. A profound revelation emerges: Korosuke, Eiichi's robot companion, is a recreation modeled after Sai's deceased son, explaining their deep connection.

Pursued by samurai forces intent on his capture, Sai needed aid to escape. Eiichi employed a teleportation tape device, enabling Sai's evasion. During their parting, Sai disclosed his full name to Eiichi, cementing their familial bond across time.