Agent E is a bald eagle which works for The O.W.C.A.. Its base is supposedly located near where Phineas's Grandparents live, as this is where its 'nest' is located. It made its debut in the episode "The Ballad of Badbeard".
Agent E came soaring down after Agent P is unable to open up one of the secret passageways to his lair located in the area. It took Agent P to its nest where Major Monogram communicates to Agent P through a small TV hidden in one of its eggs. Agent E then put worms into its nest, indicating that it is expecting the other eggs to hatch soon. It then dropped Agent P off at the docks and isn't seen from again until near the end of the episode, where it swoops down and rescues Agent P and Candace, although it may not have realized this from Heinz Doofenshmirtz's island, which had been set to self destruct.
Also it once had the duty of hand-cuffing Agent P, along with another agent, due to Perry's supposedly poor behavior over the past weeks. However, Perry tricked them into hand-cuffing themselves; he then ejected himself off the chair and the agents ran after him. ("Cheer Up Candace")