While in Kyoto, Toshimasu met and befriended Naoe Kanetsugu, Uesugi Kagekatsu's karō. The two became close friends. Consequently, Toshimasu agreed to join Kanetsugu in the Uesugi clan's invasion of Aizu. During the retreat from the failed invasion, Keiji was given the task of leading the rear guard. Riding his horse Matsukaze into battle and brandishing a two-pronged spear, he made a splendid show of force. Due in part to Toshimasu's actions, the Uesugi forces were able to retreat largely intact.
After this, Toshimasu returned to the capital and devoted himself to arts and literature. Keiji was barred from Toyotomi’s Kyushu campaign for his wild way. When the Tokugawa challenged the Uesugi in 1600, he once again fought with Uesugi’s army. In the battle against the Mogami, he broke through the enemy lines with only eight riders, and shattered their formation. After the Uesugi clan's move to the Yonezawa Domain, Toshimasu remained with them, serving as a retainer.
Keiji's armour can still be seen today at the Miyasaka Museum.
Matsukaze was said to be a horse of immense strength, able to carry his master's large frame for days. After his master's death, it is said that Matsukaze ran off and was never seen again.