The The Bones Beneath My Skin by TJ Klune

In the spring of 1995, Nate Cartwright has lost everything: his parents are dead, his older brother has disowned him, and he's been fired from his job. Looking for a new direction, he returns to his family's summer cabin in the mountains of Oregon.

The cabin should be empty. But it's not. Inside is a man named Alex - and an extraordinary little girl who calls herself Artemis Darth Vader. But there's far more to her than meets the eye. Soon, it becomes clear that Nate must make a choice: drown in the memories of his past, or fight for a future he never thought possible.

Because the girl is special. And forces are descending upon them who want nothing more than to control her.