Maybe a fox
(Junior Book)

Sylvie and Jules, Jules and Sylvie. Better than just sisters, better than best friends. Jules' favourite thing is collecting rocks, and Sylvie's is running - fast. But Sylvie is too fast, and when she runs to the most dangerous part of the river one snowy morning to throw in a wish rock, she is so fast that no one sees what happens when she disappears. At that very moment, in another part of the woods, a shadow fox is born: half of the spirit world, half of the animal world. She, too, is fast, and she senses danger. When Jules goes to throw one last wish rock into the river for her lost sister, the human and shadow worlds collide with unexpected consequences.