The Keyring Chronicles: The Heart Key:

In a world hidden from our own, people known as guardians protect humanity from threats to their day to day life. Sam is one of these guardians, but he isn't like other guardians. He's half warlock, a male magic user that is known for stirring up trouble in the world. As he struggles with finding himself, the world is in danger from a shadow long forgotten.

Summer is a girl who is just like any other girl, until the day she discovers she's a guardian, and is plunged into a world unknown to ...