When Rose was once a baby, she got separated from her parents by the Huntsman. This results in her training with the Huntsclan without ever knowing about this.
Since then, her purpose in life is to slay the American Dragon. What she didn't know is that the he is none other than Jake Long, the boy who has a crush on her. Eventually, she found out his dual identity and lost contact with him by transferring into the Huntsclan Academy.
However, she got an unexpected reunion from Jake, and is aiding his plans in stopping the Huntsclan. The only way for this to happen is to wish the Huntsclan is entirely dead, including herself. Jake saves her with his wish, but at one price: her memories of him. She moved to Hong Kong after the events.
Later, she got her memories back in the series finale to help Jake face off against the Dark Dragon.