I know some people nevereverever read profiles or journal userinfo, but if you're curious about the origins of Nuriko and don't know, I wrote it out. :D And I'll put it here for the lazier among you!
Um...but there's spoilers, so be warned. :D;;
Seven celestial seishi born with special powers for each of the four gods: Suzaku, Seiryuu, Byakko, and Genbu. Fushigi Yuugi is the story of the seven Suzaku seishi; or rather, the story of the conflict between the Suzaki and Seiryuu seishi. Nuriko is one of Suzaku's seven warriors: yanagi, the willow. The symbol appears on his chest, right above his heart, whenever he uses his power of incredible strength.
During that time, Nuriko was born in the capital of Konan (Suzaku's country, for those that haven't seen the series), and was a crossdresser for 8 years. His reason for this was because of his sister, Korin, who died when he was 10. The two were completely inseparable, and the sudden loss of such an important person in his life was so devastating that he couldn't deal with it. He decided that she should have lived, that he should have saved her, that she was more worthy. He became her, for that reason, and because he never wanted to lose her. He took her clothes, her name, and became her.
He continued doing this for 8 years, first sent to his aunt's house when his parents couldn't make him dress "normally", and then sent to the palace brothel back in the capital. He met Hotohori, the emperor or Konan, there, and fell in love. Or "Korin" did, at least. In reality, as the series goes on and Nuriko becomes more and more involved with Hotohori, Miaka, and everyone else, his feelings and behaviors slowly begin to change - he gradually releases Korin and slides into his own skin, as Ryuuen. As Nuriko.
The peak of this change happens in Hokkan (Genbu's country), during a journey, when Nuriko takes his long braid, the very last symbol of his "Korin" mask, and cuts it off. His friends are all shocked, but somehow, understood that it would happen. The man that sat with them wasn't Korin, it was Nuriko, their friend and fellow warrior.
This change happens shortly before one of the most jarring and memorable events in the series: Nuriko's death. He's the first of four of Suzaku's seishi to die, and at the point of his death, the entire series turns. It represents a change in the series, for Miaka especially - she truly begins to realize that her journey in Konan and the other countries is not a game, and that the people she has met, while they are characters in a book, are true people that feel, think, and die.
Later on, Nuriko (as well as the other fallen seishi) return in a final battle against the remaining Seiryuu seishi to protect their friends. They remain as ghosts in the physical world until the end of the OVA series, at which point they are reborn.
Mugenjou Nuriko is reborn in the modern world, along with modern incarnations of his friends (as a note, I know this kinda limits things for new people, but... :\ I got here first, sorry~!). This time, his family remains whole: his sister doesn't die, nor does he, and he is given the chance to grow up with, and live around, the people he has sworn to protect for ages. Recently, he's been forced to move to Mugenjou, MA, with his family, separated from the people he loves. While it's been hard to leave his friends behind, he is given an opportunity to meet an entirely new group of people.
Most prominent in this group is Kaoru Kamiya, a sophomore that he began tutoring in Math a while ago. The tutoring graduated to cooking lessons, and slowly, Nuriko began to realize how like Miaka she was, how charming and strange she seemed. He slowly began to care more and more about her, as she did for him. After an almost agonizingly long period of hesitation, they finally admitted this to each other.