First director of the Monchujo (Board of Inquiry) of the Kamakura shogunate and one of the Thirteen-Member Council. A minor Kyoto aristocrat, his mother was the sister of Yoritomo's wet nurse, and he kept Yoritomo informed of political developments in the capital by letter even before the uprising. In 1180, he urgently reported Prince Mochihito's decree and the Taira's plans to hunt down Minamoto survivors, directly precipitating Yoritomo's rebellion. After the uprising, he moved to Kamakura and in 1184 became the first Monchujo director, handling litigation among vassals. As a legal expert, he laid the foundations of the shogunate's judicial system and was named to the Thirteen-Member Council after Yoritomo's death. He died in 1221, after the Jokyu War.

A practical administrator well-versed in law and governance. He combined intelligence in leveraging Kyoto information networks with the administrative skill to build a judicial system—a quintessential civil official.

The Monchujo played a vital role as the shogunate's judicial arm, contributing to the development of warrior law. Without Yasunobu's intelligence reports, Yoritomo's uprising might have been delayed—making him an unsung hero of the shogunate's founding.