Agent F is a frog, his first name is unknown. He works for the O.W.C.A.. It got mentioned that he got along really well with an Agent C, a cow ("The Chronicles of Meap" character commentary).
As revealed in Where's My Perry?, Agent F is an excellent swimmer and naturally exterminates unwanted insects. Even though Agent F's first name is never revealed in the actual show, in "Calling All Agents", Doofenshmirtz refers to him as Ferdinand the Frog.