The last changer

Avia Young, Isn't very happy. In truth, she never was. She's been through foster homes since birth, but can't stick anywhere. One day, she just gets sick of it and runs away. Strangely, she started getting attacked by... Monsters. But only monsters from her big Greek mythology book. Luckily, she met a fox and named him Tar Tar. Now, why would a fox be helpful? Well, he's a changer.
Tristan Chean adoption home was burned. The Man running the place didn't notice the basilisks chasing him. Tristan outruns the beasties, but bigger ones get on his tail.
When Tristan and Avia meet, they come to the conclusion they're the same thing. But are they really?