Taigen Suufu (Sessai) was Imagawa Yoshimoto's military advisor. Suufu's father was Ihara Saemon no Jo and his mother was from Okitsu family. Owada Tetsuo has more information on Suufu and his relationship with the Imagawa in his book, Imagawa Yoshimoto. At an early age, Suufu entered Kyoto's Kenninji Temple.
His death in 1555 was a huge blow for the Imagawa family. If Suufu was still alive during Yoshimoto's Kyoto campaign, the chances were high in my opinion that Nobunaga might not have triumphed at Okehazama.
Tenka no tame!