Sumi is the younger sister of Suzu and cares a lot about her. In her younger years she's portrayed, while she's either helping Suzu with her chores or playing with her. During the war time, she's works in the Joshi Teishintai, a female 'volunteer' organization troup, in Hiroshima. In her adult years, she's portrayed as a caring, but also critical thinking character which can also be very straightforward to her sister Suzu.