Kaihime was the young daughter of Ujinaga Narita of the Hojo clan, and was known to be a brave woman as well as a beautiful one. Amidst the turmoil of Nobunaga Oda's death, the Kai province was left in a state of chaos. Ujiyasu Hojo swiftly moved to take control for himself. Little did he know, Ieyasu Tokugawa had also set his sights upon Kai province. The two soon found themselves face-to-face on the battlefield. Lady Kai accompanied her lord to battle at Shinpu Castle and aided in the defeat of the Tokugawa and Sanada armies.
Kai's most famous moment was the defense of Odawara Castle from the armies of Hideyoshi Hashiba, who had all of Japan's troops at his disposal. While Odawara was besieged, Lady Kai took charge of Oshi Castle, nearby. She used the embankments in a flood attack that revealed lands previously submerged by the water, and Hojo ambush units appeared to attack the Toyotomi main camp. General Mitsunari Ishida, commanding the Toyotomi besieging army, barely managed to send back the attackers, and Kai resisted until Ujiyasu died in battle at Odawara. She surrendered, and became a concubine of Hideyoshi.
In 1591, a year after the downfall of the Hojo, Concubine Kai was sent to quell disturbances in the Oshu region of northern Japan. Harunobu Kasai and Yoshitaka Osaki rebelled in the Kasai-Osaki Uprising, which was quelled through the efforts of Kai, Yukimura Sanada, and many others. After the death of Hideyoshi and Mitsunari Ishida in 1598 and 1600, respectively, she moved to Osaka, and in 1615 joined the resistance against Ieyasu's Tokugawa Shogunate. She died fighting, alongside Yukimura Sanada, Kunoichi, Hideyori Toyotomi, and many others.