Nat is nongendered. That is to say, Nat lacks genitals of any kind. Depending on the setting Nat could be a nonhuman, an abnormality, magically tampered with, an android, a golem, or whatever 'fits' with the world. Often though it's best just to not worry about it. Regardless, Nat has been a neuter since birth, or at least longer than he can remember, and suffers from no obvious trauma.

Feel free to use either 'he' or 'she' to describe Nat in a RP. Its totally down to personal preference so most people use whichever one they find more attractive. In most cases I will follow your lead. For the readability this description I'm going to be using 'he', but I change it monthly so feel free to use whichever you want.

Nat appears totally androgynous with moderately long white hair. Lack of hormones have left Nat looking young and undeveloped for his age. Other than the 'barbie doll'esque crotch, Nat is a normal human in form.

Nat doesn't have any particular gender identity. Usually he's content with posing as whatever gender people first mistake him for.
Nat is a frequent traveler, but whether he's been somewhere for three days or three years, he always manages to look like a tourist; Lost and out of his depth, but nevertheless curious and interested. From his travels Nat has become a decent survivalist and hardly naive. He is, however, innocent and always looking for the best in people.
Friendly and inquisitive, Nat is always interested in meeting new people, however his lifestyle means that he rarely has an opportunity to get to know any one person very well, leaving him with many acquaintances but almost no experience of true friendship or deep relationships.
Nat loves to dress up and mess around with clothing of any kind and often supplements his travelling lifestyle by making, fitting, and mending clothing. Nat is happy to wear clothing appropriate for either gender, though his outfits usually end up a little larger than life by the time Nat has finished with them.

Sexually Nat is a little clueless, having no genitals himself and no close friends or siblings to observe while growing up. Nat understands the basics of sexuality, things picked up from half-heard conversations and observations, and finds the prospect fascinating, however the real thing might prove more intense than Nat could ever imagine.
More out of the unfamiliarity of being touched than anything Nat's body is very sensitive, to the point where even a kiss or caress can send him gasping and shuddering.