Who shows up to get Percy and his mother in the middle of night?

In the first book of the *Percy Jackson and the Olympians* series, "Percy Jackson & the Olympians: The Lightning Thief", it's Grover Underwood, a satyr, who shows up in the middle of the night to get Percy and his mother, Sally Jackson.

He arrives at their apartment in the middle of a fierce storm, saving Percy from being attacked by a Fury, a monstrous creature sent by Hades to retrieve Percy.