Sumire is a young high school girl with a kind heart. She loves all her friends dearly, and becomes upset when they are troubled or in danger. She joins the Venus Vanguard in order to help Lucia fight Viruses, depite being completely terrified of them. Through the series she bonds with the members of the Vanguard, particularly Lucia. Every time she faces a hardship and finds herself too weak, she vows to get stronger.