By most people, Nina isn't considered human (or Wyndian, although races are non-existant in the BoF5 universe) at all. Her 'wings' are ventilation systems built into her back to filter the polluted air underground. She's basically a 'prototype' of sorts, and her markings indicate that she has been experimented on with the new systems.

Unfortunately, her lungs are failing her due to the massive pollution underground. She needs the fresh air of the above world, or The Sky as the inhabitants of the lower sections call it. Thus Ryu has taken it upon himself to bring Nina to the above world in order to save her life.

The question is: if Nina is a prototype with air filters on 'her' (or it, as her doctor refers to her as) back, is she really human at all? The answer is yes.

Nina was just a normal (and most likely Low-D) girl... until she was used and corrupted with an experiment, and she gained the wing-like ventilation systems she has now. And to preserve secrecy, the wings were made so it would be nearly impossible for Nina to talk. But now the new system is failing, meaning Nina is even more weakened and doesn't have much time to live. Only the fresh air of the above world can save her now.