Ieyasu grew up in the same village as Tatsumi and Sayo, and the three were childhood friends. When they decided to head to the capital together to make money for their village, Ieyasu's notoriously poor sense of direction and troublemaking nature made him a questionable travel companion—though nobody seemed to stop him from coming along.
Bandits ambushed them on the road and scattered the group. Ieyasu ended up staying close to Sayo, and together they caught the attention of Aria, who invited them to her mansion. What seemed like hospitality turned out to be a trap: they were drugged at dinner and forced to consume substances that infected them with the "Lubora disease." By the time Tatsumi and Night Raid tracked them down, Ieyasu was among the prisoners held captive there alongside Sayo and others lured in by Aria's deception.
Tatsumi killing Aria seemed to bring Ieyasu some relief, but it came too late. The disease had already progressed too far, and Ieyasu didn't survive much longer.
In terms of combat, Ieyasu wielded an axe and had likely developed similar fighting skills to Tatsumi, since both he and Sayo had trained alongside Tatsumi since childhood.
Content compiled by AnimeList.moe from publicly available sources.
