On the outside Sumire may appear fragile, but she is hiding inner strength that she isn't fully aware of. She is a nice girl who trusts easily, and that gets her into trouble quite quickly. Though initially afraid to fight, she steps up when one of her friends is threatened.
Sumire has short reddish brown hair and green eyes. She is usually seen wearing her school uniform. When she goes into beserker mode, her eyes turn red like those of a virus.
She initially meets Lucia in the very first chapter after finding Lucia's broach on the school grounds. She pricks her finger and gets a major headache. The next day, she can see evil ghosts called viruses, which surprises her, dispite having been able to see ghosts most of her life. This makes her a target for one of the viruses at the school, which starts to follow her around. Enter Lucia, who is appears and shoots the little virus and reclaims her broach before giving Sumire a flyer. When Sumire goes to Venus Vanguard, Lucia gives her a necklace that she says will hide Sumire from viruses. Lucia returns when hunting a virus at the school that is suspiciously drawn to Sumire (because the necklace actually amplifies her scent to viruses, rather than dimming it as Lucia had told her). During the fight between Lucia and the virus, Sumire gets shot by one of Lucia's anti-virus bullets, when she recovers, her eyes are glowing red and she kills the virus. After this incident, Sumire faints and Lucia brings her back to the shop and offers her a job working with her.
In Chapter 18 when her friends are talking to Lucia, we find out that Sumire and her friends are 14 years old.