Nathan Stone is Eli's brother. He's a neurologist and he's provided Eli with false medical documents in the past in order to help him continue practicing law.
Lenore: So, his father and I went outside to find Eli naked covered in feathers and chocolate syrup.
Eli: Thanks, Mom. [to Nathan] And thank you, too.
Nathan: Come on, I was ten. Just be glad I couldn't find tar.
Eli: Nate, the knight was you. An hour ago, you were chasing a dragon down Fossil Street on horseback.
Nathan: An hour ago I was here, asleep, and my horse hasn't been ridden all night.