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She is a beautiful warrior with a sharp tongue and a deadly sword. She will teach Dart much about the art of Dragoon warfare and fights beside him in many battles. There is a well-guarded mystique about her. Game Manual

Rose is the first character to appear after the game's opening FMV and the third playable character to join Dart on his journey, the other ones being Lavitz and Shana.

Though she has saved his life twice by that point and accompanies the group on their quest to slay a Dragon for the sake of the kingdom, she remains a mystery to them; she offers no information on her past, hometown or family, and they do not ask. In battle, Rose wields a one-handed sword, and as the holder of the Dark Dragoon Spirit, her element is darkness.

There's another presumably official description that is all over the internet, though I cannot find its source; it's much better than the poorly worded one in Prima's Official Strategy Guide, albeit strikingly similar in part:

The mysterious Rose is a warrior who fights alongside Dart. Nimble with a sword, she slices through the world with a knowledge far greater than most. Her mystery and allure enchant all who meet her, yet she remains alone.

Role

Rose initially acts as a guide and mentor to Dart, the party and the player. On the surface, it is her who initiates the chain of events on their journey by activating the dormant Dragoon Spirit in Dart, and she provides him with knowledge of the Dragoons, ancient weapons and creatures thought to be extinct – knowledge that becomes crucial as they find themselves caught in a plot of a scope far greater than they can imagine, starting with the previous abduction of Shana. Rose further teaches Dart how to wield his power, as in how to fight as a Dragoon, and warns him of its dangers later on.

Lavitz: Why do you know about such things? You know too much about things nobody could know.
Rose: It's better than ignorance, isn't it?
Lavitz: That's true, but yours is not mere knowledge. It's a story in a legend that has been lost. Disc 1: Hoax