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Seiya is the ringleader of the Three Lights. She first runs into Usagi at the airport when Mamoru is about to leave to the United States. She then transfers to Juuban Municipal High School, along with Taiki and Yaten.
Seiya is a grade 10 student and becomes a star American football player in Usagi's school, and also replaces Haruka Tenou as the star athlete, which causes Haruka to become annoyed. However, the Sailor Senshi becomes suspicious about her true identity. She also develops deep feelings for Usagi.
According to Takeuchi, Seiya is depicted as cool, but not very understandable. She is always mad and a worrying super-pessimist.
Seiya has a typical teenage boyish appearance. She has long black hair tied into a ponytail and wears silver earrings. She has blue eyes. As a member of the Three Lights, she wears a green shirt with a yellow tie and a red coat over it, with a red rose.
The name "Seiya Kou" means "star field light" (sei ya kou).
- According to Takeuchi, Seiya is like Haruka and Mamoru broken in half and added together. She was modeled after Jenny Shimizu.
- In the Mixx manga, her name was changed to Seiya Lights, implying to the Three Lights.
- Naoko Takeuchi made her share a birthday with Arnold Schwarzenegger.