I already know she's going to be a knight errant, making it her personal mission to try and make the world better, little by little. Maybe she's seen too much suffering in the dark, unseen corners of the world, and in alleyways, and it's touched her heart, convinced her to take up a cause.
Perhaps she was a sheltered girl, "protected" from reality's harsh truths, but after hearing how poor-off the world was, she leapt in without really thinking about it, and exposed herself, unprepared, to the darker side of the world. And rather than simply give up and crumble under pressure, it reaffirmed her belief that something needed to be done.
I can see both having potential to develop into showing aspects of the "Knight in Sour Armor" trope (which I'd probably play for comic relief).
As for why I picked an Au Ra, that actually aligns with tropes as well. Particularly about dragons. Dragons are often portrayed as sagely, if jaded, entities, older than most mortals, even when they're merely "juvenile" among their own kind. With them comes a sense of detachment, as if you're staring at the closest mortal creature to godhood. And yet, they are just as slave to emotion as any mortal man. They feel fury, they feel desire, and they feel weakness, just as any sapient creature does.
Black dragons tend to be more aggressive, violent, and passionate, embracing their desires with great fervor. This embrace of emotion would also be reflected in my character. She doesn't have any filter. Sure, she might shy away from instantly admitting she finds someone attractive, but she trusts her heart to lead her true. Her passions may not be as intense as the true black dragons, but I intend to make the connection notable.
Some other dragons tend to be great scholars, thinkers, learners. They do not delude themselves into thinking they are omniscient, but they are far from ignorant. If friendly, they tend to be mentors, and are relatively humble. Otherwise, they are haughty and arrogant at minimum, and downright greedy and spiteful at worst. My dark knight would show this same love of learning and life. Again, not to the same extreme as true dragons, but enough so that the connection is visible.
As for why she's intended to be a Dark Knight, there are two reasons...
One, I have always enjoyed taking a "negative" concept and introducing it to a "positive" character. Dark Knights, mechanically, show quite a bit of self-sacrifice in their abilities, as last I checked, many of their skills incurred an HP cost. This may have changed, I may have been flawed in my research, or I may be right. But it would be nice to show that self-sacrifice attribute in a character, having them be a living example of both "Dark is not Evil," and a variant of "Bad Powers, Good Person."
She is also an example of "Good is not Soft," or at least, I hope to make her one. She might give sapient foes a chance to change their ways, and prove they are capable of good things. But she only gives them one chance to do that.
As for a more solid, and quite likely far more attainable goal, she does want to live to see the Garleans driven back away from the homes of Eorzeans. She'd be okay with the Garleans still being an empire. So long as they stayed on their end of the world.
As for how her race influences her interactions, she tries to ensure it doesn't. At times it does anyway (horns make for an interesting conversation piece after all), but generally, she as a character tries to keep race separate from her interactions with others. Her beliefs, her philosophies, are how she copes with the world. When she sees the world through a lens of wisdom, both formed from her own experiences, and borrowed from old friends and family, it seems less alien to her.
Generally she's not exactly a simple character. While her goals are very morally-grounded, the experiences that shaped those goals are a long tale indeed.