Shukumaru resides in a war-torn Sengoku-period village, acting as both protector and scavenger who gathers vital supplies from battlefields for the community. His role as defender is cemented when he rescues Suzuko from attackers. Exhibiting a brash and straightforward personality reflective of his era, he openly declares his intent to marry Suzuko soon after bringing her to the village.

Unknown to him, his true origin lies in modern times. He was originally a young boy named Shu, living near Suzuko in contemporary Japan. The same gas explosion that transported Suzuko flung him approximately ten years further into the past, where he grew up fully immersed in the Sengoku period, leaving him without memories of his modern life. Physical proof of this past surfaces in a distinct appendectomy scar, identical to one Shu once showed Suzuko. Later referred to by Shukumaru with the childlike term "apple-citis," this scar becomes crucial for Suzuko to recognize his true identity. Its revelation also dispels earlier tensions about a potential sibling relationship, confirming they share no biological ties despite originating from the same modern adoptive family.

Shukumaru's development involves a shift from seeing Suzuko merely as a companion displaced in time to recognizing their destinies are intertwined through time loops triggered by fire. After a temporary return to the modern era, he chooses to journey back to the past with Suzuko, solidifying their romantic relationship and shared future within his adopted time.