Questions for you!
1.) Can Aerith use Magic on/around your character(s)?
2.) Does your character use Dark Magic? (Aerith is a White [Magic] Mage, so I expect she would be able to detect Dark Magic. It would likely make her awfully uncomfortable and maybe even defensive depending on the situation.)
3.) Would your character be interested in protecting a flower-girl? (She is much more than she seems. Much, much more. If you would like to know about her abilities, look below.)
Aerith is considered a Final Fantasy White-Mage. As such, she is more of a Healer than a Fighter (i.e. she relies on Magic more than her physical might). That is not to say, however, that she can't use her staff in battle (because she can). However, Aerith's true might lies in her Magic. She can also use Summon Materia to complement her Magic and make it even stronger.
Her Ultimate Limit Break is called Great Gospel and she has only used it once before.
She is half human and half Cetra/Ancient. Cetras were amongst the first beings on Gaia. They only employed White Magic to protect the planet and stayed away from Dark/Defensive Magic entirely. As such, when humans surfaced, they were, more or less, easily defeated because of their peaceful natures.
Aerith is the last Cetra - or, rather, she is the last partial Cetra. Her mother, Ifalna, was actually the last full Cetra, but she died when Aerith was very young.
As such, Aerith is incredibly powerful, but it's limited to helping others/aiding the planet.
If you have further questions, please don't hesitate to ask them here. :)