@D u b b y It's kinda hard to explain but the character Nefelpitou's gender is extremely ambiguous. As you can see, Pitou has a really feminine form but the author tends to descript Pitou as "him", sometimes as "her". So the fans are confused by his/her unidentified gender.
To elaborate on Mantama's post, the japanese don't tend to state gender directly. When speaking about someone, they use an inflected version of "that person" rather than him or her. So in this case, Nefelpitou is "that boyish person", which could mean tomboy. In the manga, Pitou is also shown with a bust sometimes and a masculine figure sometimes, furthering confusion. Most fans will tell you to consider it's gender whatever you want it to be.
@ Spectrallynx - If I see it I will, but I'll warn you now I am more of a rabid Pitou fan than a rabid HxH fan. That said, I will keep an eye out.
@ Chunkeh - In this dou, Pitou is 100% female. It would be false advertising to throw in a catboy tag when there is no catboy, regardless of the HxH backstory