I like her attitude and personality - serious and intelligent, but playfully bratty at times. She kept things to herself well, and was pretty considerate of how others would react - Tifa, for instance. I like the way she isn't "one person" all the time - depending on the situation, she can be stern and motivated, or light-hearted, or even cheeky and impetuous as appropriate. A good balance, I think.
Also, I consider her one of the most genuinely "heroic" FF characters - she went on a dangerous journey, leaving home for the first time in her adult life, because of the chance to confront/discover her past. Once she learned the truth, she literally gave everything to do her best for the planet.

Actually, she's one of the better characters out of all the games, in my opinion.