Nyaf Transgender Lore:

Nyaf was and is currently sexless, and was also genderless before making his way onto Earth. Calling him the N.Y.A.F. and an “it” would be more accurate when it comes to referring to the him before he crash landed into a bank vault. The N.Y.A.F. managing to hit its head made his brain “reset” to a childlike disposition, and a state of mind that’s less artificial and more organic. If things happened only slightly differently, Nyaf likely would’ve gained a new identity, being that of a girl. Nyaf’s old personality/identity (both are relevant) as the NYAF is essentially lost, having forgotten the soldier mentality indoctrinated into him with the amnesia that made him forget his purpose for being on Earth. Basically, our Nyaf is not the same as the one before he came to Earth because they’re two different people in a sense. Had Nyaf identified as a girl, she would’ve used her Super-Proportionalism to grant herself a more feminine appearance with a suitably shaped body. And we probably would be calling her something like Nyaffie instead.
N.Y.A.F. doesn’t stand for Not Your Average Fish by the way, but Nyaf thinks it does. He’s completely forgotten what it really means, so not even I know.

This brings up the question surrounding the sound of his voice. Nyaf could use Super-Proportionalism to alter his vocal cords to make them produce more masculine sounds, but I doubt he actually cares about that. I’m willing to bet that his voice is androgynous at least, or simply unclear.