More like "???". It's never specified anywhere and they're so skinny and androgynous that trying to eyeball a guess is a wash, doubly so with the rather stylized artstyle.
Far and away the least of issues with that kid anyway. Their mom, the witch Medusa, is right up there with Gendo Ikari and Ragyo Kiryuin in Top 10 Caring Anime Parents so... yeah.