*WARNING CONTAINS SPOILERS!* This is a four book story. Secrets of the Sirens, The Gorgon's Gaze, Mines of the Minotaur, and the last, The Chimera's Curse. The quartet is by Julia Golding.
It was so intense, though at first the story seemed a little well...slow, but it got more interesting. Connie learns that she is a rare person, called a Universal Companion. She can be a companion to all mythical creatures.
In the four books she battles the enemy, Kullervo, who wants to rid the world of humans. He is a shape shifting mythical creature who is the Universal's companion.
In each one she fends him off and he leaves for a while, but he always comes back more powerful, and in the end she battles him.
And to see what happens you'll have to read the books! :)