惜別の向日葵 (Sekibetsu no Himawari)

Background
The story centers around a pair of siblings during a time of war. After their father dies in duty and their mother commits suicide, they are left to care for themselves. Eventually when the brother is old enough he's sent to the war as well and laments that he would've at least liked to see the sunflowers they planted grow again, having gotten so used to seeing them every year. When the sister, known to cling to the past, begs him to stay he comforts her and swears to fight for her future. However the brother dies, and the sister lives on alone and continues to plant the sunflowers each year well in to her adulthood.

This song was released on VersuS.