Cold Spell by Jackson Pearce

Kai and Ginny grew up together--best friends since they could toddle around their building's rooftop rose garden. Now they're seventeen, and their relationship has developed into something sweeter, complete with stolen kisses and plans to someday run away together.

But one night, Kai disappears with a mysterious stranger named Mora--a beautiful girl with a dark past and a heart of ice. Refusing to be cast aside, Ginny goes after them and is thrust into a world she never imagined, one filled with monsters and thieves and the idea that love is not enough.

If Ginny and Kai survive the journey, will she still be the girl he loved--and moreover, will she still be the girl who loved him?