Also known as Ienari. Ishikawa was only 13 when he first served Tokugawa Ieyasu, who at the time was a hostage of the Imagawa at Sumpu (Suruga). After the Battle of Okehazama and the death of Imagawa Yoshimoto (1560), Ieyasu became independent, and Ishikawa rose to become one of his chief councilors. However, Yoshimoto's successor Ujizane still held Tokugawa's wife and son hostage at Sumpu. While Ieyasu plotted a way to have them released, Ishikawa went to the Imagawa capital to act as their guardian, a dangerous assignment. Ieyasu did work out a deal whereby the hostages were released, allowing Ishikawa to escort them home in safety.

In addition to being a skilled administrator, Ishikawa saw active service in a number of famous battles, including Anegawa (6/1570) and Mikatagahara (1/1573). In 1583, following the death of Oda Nobunaga, Toyotomi Hideyoshi and Shibata Katsuie came to blows; after Hideyoshi's victory Ishikawa was sent to present him with Ieyasu's congratulations. The next year, Tokugawa decided to take issue with Hideyoshi on behalf of Nobunaga's 2nd son, Nobuo; Ishikawa and Sakikabara accordingly issued statements attacking Hideyoshi. Ishikawa served at Ieyasu's Komaki headquarters during the resulting Komaki-Nagakute Campaign. Following the cease-fire, Kazumasa abruptly switched sides. Evidently dismayed by what he took to be Tokugawa's foolhardy path of resistance to Hideyoshi, Ishikawa left for Hideyoshi's court and entered his service. His departure from the Tokugawa camp proved quite inconvenient for Ieyasu, who was obliged to restructure his defensive policies and military organization, owing to Kazumasa's intimate knowledge of the Tokugawa.

Kazumasa's son Yasumichi (1554-1607), also a veteran of the Komaki Campaign, stayed in Ieyasu's service and was given a fief in Kozuke in 1590 (Naruto, 20,000 koku). After the Sekigahara campaign, he was transferred to Mino (Ogaki, 50,000 koku).

Compiled by F.W. Seal