Contrary to the above mentionings, we know she's a he. We just think she rather be referred to in the feminine sense. If you already know the following, just skip ahead. Nuriko once had a younger sister, named Kourin. Kourin died while running after Ryuuen. She was hit by a cart as she crossed the street. Nuriko, of course, blamed this on herself and decided to keep Kourin's memory alive by dressing and acting like her. In essence, letting Kourin live through herself. I have no idea what her parents thought about this, but she somehow later ends up in Hotohori's court as a princess of the realm :), thus entering the story of The Universe of the Four Gods. Later on in the story, after Soi's entrance in the anime version, she tells Miaka and Tamahome the story of her childhood. Not long after that, during the search for the Shinzaho (and after some rude guy's comment in a bar), she decides it would be better to start living as Ryuuen (a man, that is), chops off her hair, and promptly falls in love with Miaka. (What is it about that girl?)