She tries to save everyone. She has always tried to, no matter what the cause of it will be for her.
She was born with the strength to do so, and so she does. Saving people was not even an option for her, because it is what she just does. One might think that it was easy for her because of her overwhelming strength, and it is—the times that the odds were against her almost did not exist.
She wasn't a fool, she knew that she could not save everyone; but that didn't matter. As long as she saves one more life, she would. For hundreds and hundreds of years, she did…
…until the dark days came for everyone's lives.
And now, once again, a dark day looms for her.
Themarians do not really need to breathe, they just do so because it feels nice to do so—but right now, try as Aerith might not to breathe, she wasn't able to do so as her heart, which also rarely beats, was crushing her chest with the way it drummed.