Ryouko Tachi (舘涼子, Tachi Ryōko) is a second year student at Shiratori Girl's Academy, all girl high school. She first becomes a friend of Kaoru Sugimura when they meet during an early morning workout. She is also a rival of Nana Chigusa, having previously lost an 800m race to her.
She meets Kaoru and helps him when Kaoru has problems while he is out running one morning. Due to her slim build and short hair, she is mistaken for a boy, by a classmate of Nana's and even by Kaoru himself.
Kaoru mistakes her for a boy until she mentions participating in a heptathlon. Annoyed at the misunderstanding and/or her lack of an obvious chest, she shows him her modest cleavage as proof of her gender. After learning of Kaoru's secret hobby, she develops an interest herself and tries to have Kaoru involve her in some of the activities.
Unlike most characters, she seems to be able to switch from being a submissive to a dominant, but is clearly more appreciative of her submissive side. It is implied that she also loves Kaoru, but is aware that he will never see her in the same way as Nana.
The best girl.