Born "Toramatsu," in Iidani, Totomi in 1561, Naomasa's father Naochika was killed for suspected treason against his lord, Imagawa Yoshimoto. Forced to flee, Naomasa was raised by families other than his own, until he serendipitously attracted the attention of Ieyasu while the later was hawking in Hamamatsu, and was taken in by him in 1575. Not only was he and his family forgiven, but they were given back their old stomping grounds of Iidani.
Naomasa distinguished himself in several battles and episodes. He fought his first battle in 1576 against Takeda Katsuyori at Shibahara, in Omi. In 1582, he assisted in Ieyasu's escape from Iga, and in the same year fought at Wakamiko against the Hojo. At Nagakute, Ii defeated Ikeda Tsuneoki, fighting so fiercely as to elicit praise from Hideyoshi, who was on the opposing side.
As Ieyasu made his way into Edo in 1591 following his appointment there by Hideyoshi, Naomasa was given Minowa Castle in Kozuke. With this came 120000 koku, the largest amount among the Tokugawa retainers. By the time of the battle of Sekigahara, Ii Naomasa was considered one of Ieyasu's "Shitenno," or top retainers.
Preceding the battle, Naomasa made his way west with the forces of Honda Tadakatsu. He again distinguished himself by marching past the vanguard of the eastern army, and making the first attack of the battle. Naomasa and his "Red Devils" would fight valiantly, mainly against the Shimazu, from whom Naomasa would receive a bullet wound. This wound would be named as part of the cause of his death 2 years later. Before his death, Naomasa would receive Ishida Mitsunari's castle Sawayama in Omi.
