Hmm, OK, well, what I'd like is for her to be able to contribute usefully to the final battle (and possibly prior to that), and to be generally awesome in doing so. I don't expect her to be uber-powerful, she's only going to have a couple of weeks to learn to fight, but she is a smart girl, and her magical affinity is anti-spirit, so I'm sure she can think up something that will at least assist her side (along with being Rider's master, of course, at least at the end).
I would imagine she'd develop over the course of the route from not wanting to fight and not wanting her sister and Shirou to know that she's involved to being determined to protect them in any way she can, regardless of her fear that they will reject her if they learn she was secretly Rider's master all along.
