A better theory for the movie would be that it follows anime convention, thusly:

Some energetic, carefree girl shows up at school, and the substitute shinigami crew take notice because of her enormous spirit energy. But beneath this bubbly, happy-go-lucky exterior is a dark past that she thought she had escaped from. Eventually, some badguys from her past (who've been searching everywhere for her because she holds the secrets to untold power) catch up and Ichigo and crew have to help their new friend, even though they aren't entirely sure of her intentions. There will be a love-triangle twist between Ichigo, Rukia, and the new girl.