Kotoko Sumiso is a high school student with shoulder-length light pink hair cut in square bangs and coral-pink eyes. While wearing a yukata, she ties her hair up and secures a flower within it.

Her friendly and polite personality leads her to approach Seri Koyuki, expressing her long-held wish to converse with him. She observes that Seri once seemed angry and unapproachable but has recently grown more animated. Kotoko initiates summer break plans, suggesting they attend a fireworks display or visit a destination together, and arrives at Seri's house in a yukata to discuss these ideas.

She provides Seri with her phone number and email address to sustain communication over the break, signaling her interest as his love interest. Her interactions with Seri are frequently interrupted by Kabuto Hanadori, whose presence slightly repulses her.