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Seiya is the leader 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 then a grade 10 high school 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.
Throughout the series, she also develops deep feelings for Usagi. However, Usagi was still deeply in love with (and in Stars, mourning) Mamoru. Despite this, the two came to understandings about each other and their missions, and appeared to still care for each other.
Seiya is also deeply devoted to Princess Kakyuu.
According to Takeuchi's Materials collection, Seiya is depicted as cool, but not very understandable. She is always mad and a worrying super-pessimist. Though, Seiya appears to develop as a character throughout Stars, as well.
Seiya initially 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).
- The Japanese school year begins around April 1st and ends in late March of the following year. Because Seiya's birthday occurs on July 30, Seiya ages to 17 about 3-4 months after the school year begins in Stars, which is not very long after the school year begins. In comparison, Usagi ages on June 30th.
- Seiya's birthday occurs only one full month after Usagi's exactly.
- 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.