Well me and F3d were doing a little bit of searching and F3d found Mateo's Flickr page with this image:

Look at the girl on the right
now that image was Taken in 2011 meaning its about 3 years old, meaning Pie would've been about 12-13 at that time
now lets compare this to the original picture Pie posted of herself

Looks about the same.
similar hair color, fairly similar skin tone, same earrings, and most of the facial features are the same.
Judging by this I'm pretty sure we can confirm that pie is indeed a female.