Washio Sumi wa Yuusha de Aru: Gin-iro no Kioku

Silver-Colored Memories is a bonus story released with the Blu-ray of Washio Sumi's Chapter - Spirit. It takes place during the events of Washio Sumi is a Hero and features Sumi and Sonoko helping Gin prepare a birthday present for her younger brother. (Source: Yuuki Yuuna wa Yuusha de Aru Wiki)