Basically what said, because said old FEfags like to pretend that Lyn didn't exist or that they weren't totally infatuated with her in the same way (or Nino for the pedos). You could even make an argument for FE6 since Roy was basically a harem protag. Other than that, the lewd outfit, the 'problematic' nature of her in general and the overexposure in Heroes are all things that come together to make her the most controversial character in the franchise. That IS doesn't give a fuck about doing anything with her as a character outside of her game doesn't help, even though there's the potential for it.
Yea Forums is actually about the only place where she seems generally well-liked, or at least that you can be open about it without everyone turning into shrieking monkeys.
I'm inclined to disagree within Fates at least. If nothing else she had more going for her than any of the other 'important' females like the female royals, Azura and Felicia.
