Perry the Platypus

He is Phineas and Ferb's pet platypus, who, unknown to his owners, lives a double life as a secret agent. He has a lair under the Fletcher's house that can be entered through a lot of passages, like a tree in the backyard the "Chimneylator", (an elevator inside the chimney), or lots of other places that lead to underground tube systems. His nemesis is Dr. Heinz Doofenshmirtz, and he is always sent to foil the doctor's plans, which he always successfully accomplishes. A running gag in the show, is that his missions always end up covering evidence of Phineas and Ferb's activities by accident, thus Linda never finds out about the boys' schemes.