So is the long haired person a girl or boy....?

I am pretty sure it's a girl

I'm leaning more towards boy who for some reason had to be a 'girl'

but I don't know

Late reply, but I hope this helps you.
http://www.mangago.me/read-manga/slightly_sour_secret/nbt/716e72c39ffe3a3b/Vol1_Ch10/9/

Scroll down and it's a profile of the MC.