Ryoko is the probably the main main character in Tenchi Muyo (besides Tenchi and Ayeka). She first appears in episode #1 when Tenchi accidentally resurrects her (sort of accidentally on purpose, really.) You see, Tenchi got the keys to the Misaki shrine almost by accident (and under rather humorous circumstances) and when he went to investigate the shrine, he resurrected the demon, Ryoko. It turns out that she isn't really a demon, but an infamous space pirate and a criminal who's wanted throughout the galaxy. She was locked up in that cave for seven hundred years after she had attacked the planet Jurai and fought Yosho, Ayeka's half brother. He went off to pursue her and fought her all the way to Earth; when he defeated her he locked her up in the shrine and stayed to keep watch over it... but that's another story as well.
Ryoko is in love with Tenchi (yeah, like which of the girls isn't?) but he seems sometimes like he's in love with her a little too. Ryoko always gets in a fluster if she catches Ayeka flirting with Tenchi (which she does often, even though she claims not to) and is always getting into fights over Tenchi (and everything else they can argue about) with her. In one of the Time and Space Adventure episodes in the Tenchi Universe series she and Ayeka turned out to be in love... but I won't go there.
Ryoko is the most frank person about sexual things in the series; second most frank is me, Washu of course. (After all, I did offer to bear Tenchi's child, didn't I?) Ryoko's always trying to seduce Tenchi or get into his room, or burst in on him when he's in the bathroom... etcetera. She's a bad girl with, well, not necessarily good intentions, she's just in love!
She's a thief and a fugitive to some extent, but other than that, she's a good person. Well, not really, but we all love her anyway.
She's the daughter of me (Washu) and a creature called Mass. I created her by combining the Mass with my egg cells. But I treat her more like my daughter than an experiment.
Ryoko (or Ryouko) sort of owns Ryo-Ohki, who can transform into a spaceship/crystal, but she's also sort of like Ryo-Ohki's sister. See, Ryo-Ohki was also an experiment of Washu's, only Washu used a mineral life to create her. Bolth Ryouko and Ryo-Ohki have psychic powers, but Ryo-Ohki's are much weaker then Ryouko's. They bolth inherited this trait from Washu, so all three of them can communicate by telepathy.
Ryoko has, well, 'super powers'/magic that she uses a lot of the time, you know, flying, shooting lazers at people, disappearing somewhere and reappearing somewhere else- that sort of stuff. That would make sense, considering she's half goddess.
In the TV series she has a few whole episodes devoted especially to her. I like 'No Need For Ryouko', it made me cry and got me all worried at the end. You?
Well, that's basic Ryouko. I'll add on more in a while.