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. She generally tries to maintain this personality even while trapped inside Heavenly Host, though the grim reality of their situation nearly causes her to suffer from an emotional breakdown several times. She is the most bold of the girls. She is extremely close to Naomi, the two of them being best friends. She is 16 years old.