Well, there's not much on her except that she's a bounty hunter after ryoko. She's supposedly only after the money, but I believe that she has a personal grudge on Ryoko. I think that she also wants to fight against her to see how she measures up. (why else would they "challenge" each other all the time?)

Ryoko and Nagi are obviously of the same species (blue spiky hair, cabbits) or maybe it's a paradox . . .I don't know, I can be wrong (rarely ^.^)