i reckon hes human.
remember one of the possible response in her conversation with him was
"still human enough to feel betrayal?" (concerning visas)
and, if you really increase the brightness of your monitor + the brightness of the game to a degree, nihilus has quite a humanly shaped head behind the mask. and, there was a cut menu in the game featuring the twin suns with nihilus and and gawking cannok. that wouldnt work if he wasnt a human/close to human!
get the hilarious mod here: