Seika and her younger brother Seiya lived in an orphanage for a few years, until Mitsumasa Kido separated them. He took Seiya away to train to become a Saint, saying that Seiya could not see his sister again until he brought back a Bronze Cloth. Unbeknownst to Seiya, Seika followed her brother all the way to Greece. She fell while trying to climb up a cliff, and the resulting head injury caused amnesia. An old man took her in, and she lived for years in the village next to Sanctuary, remembering nothing of her brother.
Aquila Marin finally discovered her while Seiya was fighting in the Underworld. During their battle, Thanatos read Seiya's heart and saw his desire to see his sister. As revenge for managing to wound him, Thanatos decided to kill Seika in front of Seiya before finishing the fight. He attempted to attack her psychically, but Marin, Shaina, Jabu, Ichi, Nachi, Ban, and Geki shielded her from Thanatos' attacks. Seika could feel Seiya's Cosmo, and her memory returned. Seiya's desire to reunite with his sister is what gave him the strength to keep fighting until Hades was defeated.