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 not without a responsible side, however, which she forced herself to develop after her mother disappeared so that she would be able to support her father and care for her three younger siblings. She easily becomes upset whenever anyone takes her feelings towards her mother's disappearance lightly, which causes a rift between her and her best friend Naomi, when the latter does just that. Although such, Seiko is very forgiving as she cares for Naomi too much to be upset at her for too long.