Born in 1536, Sakai Taneharu was lord of Toke Castle and head of the Toke-Sakai clan. At the 1564 Second Battle of Konodai, though mobilized for the Later Hojo, he fell under suspicion after delaying his deployment and defected to the Satomi, rescuing the retreating Satomi Yoshihiro. Toke Castle then became a repeated target of Hojo attacks, which he withstood with support from Uesugi Kenshin and Satomi Yoshihiro, until surrendering in 1576. During the 1590 Siege of Odawara the clan, then Hojo-aligned, was besieged and stripped of its domain when the Hojo fell.

A pragmatic local lord who prioritized realistic calculation over blind loyalty, yet records also show tenacious resistance sustained for years before his eventual surrender.

Remembered in Chiba's Midori Ward as the last head of the Toke-Sakai clan who defended Toke Castle, the clan's seat for over a century. The ruins are preserved as Toke Castle Ruins Park.