Born to a domain physician's family in Choshu, he married Fumi, the younger sister of Yoshida Shoin (later the wife of Katori Motohiko). One of Shoin's most brilliant disciples, he was ranked alongside Takasugi Shinsaku as one of the 'twin pillars of the Shoka Sonjuku.' After Shoin's death he actively led the sonno joi movement in Choshu, spearheading the bombardment of foreign ships at Shimonoseki. He drove the radical expulsion movement aggressively, but after the coup of August 18, 1863, Choshu was defeated by a Satsuma-Aizu coalition and expelled from Kyoto. In July 1864, seeking to retake Kyoto, he commanded Choshu forces in the Kinmon Incident—the assault on the imperial palace—but was defeated. Gravely wounded, Kusaka took his own life within the Takatsukasa mansion. He was twenty-four. His death—a young life burned bright—symbolized the end of Choshu's radical expulsion movement, and also served as a turning point that prompted the next generation of Choshu samurai such as Takasugi Shinsaku and Ito Hirobumi to shift to a more realistic path.

A passionate and pure man of action. He possessed the courage and directness to give his life for his convictions, literally embodying the teachings of his mentor Shoin. His death at such a young age continues to move people in later generations.

As a hero who embodied the spirit of the Shoka Sonjuku, he is a symbolic figure of the sonno joi movement in Choshu and Japan. His heroic death in the Kinmon Incident marked a turning point in the movement to overthrow the shogunate, opening the road to the Restoration.