Sumie Nishino is married to Icchou Nishino, and together they are parents to Mari Nishino. The couple runs two businesses: a delivery service and a boarding service. Following his mother's abandonment, they took in Takeru Takemoto during his childhood, raising him alongside their daughter Mari; this makes Takeru both their nephew and their ward.

Sumie possesses brown hair, typically styled at shoulder length, and black eyes. Her physical appearance aligns with her role as a maternal and stabilizing presence within the Nishino household. Within the narrative, she represents a grounded, domestic force, contributing directly to the household's functioning through her work in the family businesses. Her character provides a counterbalance to the story's more fantastical elements, embodying themes of familial responsibility and everyday resilience, reflecting normalcy amidst the supernatural events affecting others.