A succubus is female. The male version is called incubus. So it's a girl.
Because of the new chapter's title I'm not sure anymore.
He/she explained in the previous chapters, they are born without gender, when they reach certain age they need to find a human for resources, and depends on the human's gender they develop male or female parts. (Being a fudanshi myself this explanation sounds so hetero (￣へ￣）)
Jesse chose the doc, but the doc didn't choose her/him, so he/she is not a boy or a girl, yet...
Is that blond kid a boy or a girl? Is this turning in to a harem? I really hope it doesn't.