I considered that too, but... I thought he'd have mentioned her by name, if that were the case. Every other woman he ever talked about was either Yuna or his mother. Could've been the reason she was able to become a Fayth relatively easily, too - she was much like an Aeon already, after all.
Well, I've only played through FFX once, you've undoubtedly got a better memory for the details than I have. I'd not be too surprised if my guesses are completely off-target.