Seiko is an extremely cheerful and energetic girl who enjoys playing around with her friends. She often comes across as immature and crass, and makes over-the-top outbursts that surprise everyone around her, but her friends are able to put up with it.
Seiko is very close friends with Naomi, whom she views as a kindred spirit of sorts since they had both lost one of their parents at some point in their lives. The two live next to each other and walk to school every day together, and like to talk about their futures together. As part of her more childish, out-of-control personality, she occasionally likes to fondle with Naomi’s breasts, and comments on Naomi’s butt as flirting, but Naomi is able to put up with it. Her feelings for Naomi are heavily implied to extend beyond mere platonic love to the point of romantic attraction, but she keeps these feelings a secret from Naomi, understanding and even supporting her feelings for Satoshi.
She is accidentally killed by Naomi when Naomi became afflicted by the Darkening and hung her in the girl’s bathroom without her even knowing.
She also appears before Naomi one last time to confess her own feelings and asks for Naomi to return them if she chooses, prompting the two to kiss.