Seiko Daichi is the mother of the Daichi family. Her personality is deeply flawed, marked by vanity, self-absorption, and a volatile temper. She acts mean-spiritedly and occasionally sadistically, particularly towards her husband Mamoru, whom she constantly belittles and threatens with divorce. Financial gain strongly drives her decisions and is a primary concern.

She neglects family duties, forcing all household chores onto her daughter Nozomi without help or thanks. Mamoru receives similar disregard, openly ignored despite his efforts. Her son Dai sometimes benefits from her affection, though this remains inconsistent. At work, she projects a facade of professional competence, though colleagues witness her temper flare when situations don't meet her expectations.

Conscripted as Earth's defenders, Seiko's initial reluctance is overcome by the promise of payment per successful mission. In combat, her personal vehicle transforms into a flying support craft; the Guide Beam creates navigational pathways for teammates, while Compact Mirage uses oversized cosmetic mirrors to deflect enemy energy attacks. Mission costs frequently anger her, as equipment expenses often exceed payments, increasing the family debt.

During the series, she has an extramarital affair with a coworker named Hayakawa, which she ends before the conclusion. Beneath her harsh exterior lies genuine concern for her family's safety during defense operations. This underlying care becomes more visible as the series progresses, with her hostile attitudes gradually softening. Her development culminates in a stronger commitment to preserving the family, moving from initiating divorce proceedings to actively participating in its reconciliation.

She has light brown hair and brown eyes.