The shy friend

Kate believes she is really lucky, at least compared to some kids at her school
Who have no friends and are alone all the time, because Kate has Sally her best friend in the entire world. Kate and Sally have been best friends ever since the 4th grade and now it's the first year of high school.

Except, Sally has always been a little odd.... For example her appearance has been pretty much the same since grade 4, in the 4th grade she seemed a bit taller than everyone and a bit more developed (puberty wise). But now she seems shorter and a little underdeveloped, and she has always creeped other people out for some reason but she seems normal to Kate?

Kate try's to believe Sally just has a disorder or something but for some reason she just does not believe that. Once Kate even thought she saw Sally in her room at 1am. Everyday Sally is getting stranger and Kate is starting to get creeped out.