I found out... this book

 

Summary from Goodreads

An idle glance through the window of a bus changes everything for Katy Rivers.
A mysterious girl, with luminous green eyes, stares back and from that moment on she haunts Katy and appears to know her every move, even what she is thinking. What is the strange connection between them? And what is the power of the emerald pendant which the girl bestows on her?
‘I’m everything you’re not and I’m going to take over your life… I’m your worst nightmare.’
These chilling words mean that Katy is gradually backed into a corner, alienated from everyone she holds dear, even her own mother. Only her best friend Luke, keeps faith and together they must find a way to defeat the girl who is determined that she will be with Katy...even until death. 

I really like this summary, but I don't like the covers. This has to be a "Don't judge a book by its cover" 
Well, I have to try it!