Define "villain." What you've got sounds like a Garlean-sympathizer, not an out-and-out destructive malevolent force. She's not trying to destroy Eorzea, she's just considering that maybe Garlemald isn't so bad. It didn't help that Nael was insane and trying to LARP Majora's Mask with us, but not everyone is out to destroy! She's tapped into that. It can be a positive thing to roleplay.
Consider that espionage isn't such a simple thing, though. Once you get past the fact that if she judges the wrong Eorzean as reveals herself, she's liable to be executed for treason you also have to bear with the possibility of Garlemald not quite believing her, either. In order for her to get in touch with the Empire to begin defecting, she'd need contacts. I suppose there's nothing to stop her from just walking up to a Castrum unarmed and explaining herself, and there are plenty of true-to-life examples of people doing just that, but those sorts of actions never result in any sort of power. Your character could end up simply finding sympathetic assets for the Empire to perhaps contact through their own networks.
There's a lot of room to work with if you go that route. It helps keep things grey on the morality scale without outright throwing down and becoming a traitor. Thoughts are dangerous things, of course, but when someone you know and trust says to you "Hey, maybe things wouldn't be so bad...?" you're more likely to get interesting conflict than outright being a double-secret Garlean spy traitor. That's done to death already.
Consider that espionage isn't such a simple thing, though. Once you get past the fact that if she judges the wrong Eorzean as reveals herself, she's liable to be executed for treason you also have to bear with the possibility of Garlemald not quite believing her, either. In order for her to get in touch with the Empire to begin defecting, she'd need contacts. I suppose there's nothing to stop her from just walking up to a Castrum unarmed and explaining herself, and there are plenty of true-to-life examples of people doing just that, but those sorts of actions never result in any sort of power. Your character could end up simply finding sympathetic assets for the Empire to perhaps contact through their own networks.
There's a lot of room to work with if you go that route. It helps keep things grey on the morality scale without outright throwing down and becoming a traitor. Thoughts are dangerous things, of course, but when someone you know and trust says to you "Hey, maybe things wouldn't be so bad...?" you're more likely to get interesting conflict than outright being a double-secret Garlean spy traitor. That's done to death already.