The Demon in the Golden Mask by K. King
The story is based around a really weird girl Sam who sees people as masked figures hiding behind secrets instead of actual people. And because of this she finds that she hates talking to them. The only person she likes is Shane, her best friend since childhood who always tells her the truth so that she won't hate him.
But he IS hiding a secret from her, one that isn't safe for anyone to know. And Sam knows he might be keeping a secret, but she doesn't know what it is until Shane's dead body is found by his parents.
Heartbroken and confused Sam tries to piece things together. At Shane's funeral she tries to escape the crowds of people and accidentally falls down a flight of stairs to be rescued by a strange boy who looks a lot like her best friend. But before she can get anything out of him he disappears.
Later before the funeral has ended, Sam hears her little sister Kitty, who may or may not have some strange abilities of her own, screaming bloody murder. Running to her rescue Sam find the boy from before trying to reach Kitty, who is screaming the name 'Loki' a name Sam has heard her say before.
The boy admits that he is Loki, and that her sister is something special, that he wants. When Sam refuses to give him to her, he demonstrates his 'power' by compelling her dad to leave them alone.
Confused Sam is thrown into his world where he is a demon and the town is in danger from his bringing back from the grave. Sam has to protect her sister, and when Loki starts getting more involved in her life, nothing is as easy as it was before.
The story is all about Sam finding a way to trust her friends, while at the same time protect her sister and the town from Loki, who may or may not want to get her on his side ;). I would give it a read for sure if you get the chance :)