Dr. Heinz Doofenshmirtz, often called "Doof" or "Dr. D," is a fictional cartoonishly evil scientist from the animated series Phineas and Ferb, created by Dan Povenmire and Jeff "Swampy" Marsh and voiced by Povenmire. Hailing from the fictional European country of Drusselstein, he operates Doofenshmirtz Evil Incorporated with the goal of conquering the Tri-State Area using elaborate inventions ending in "-inator," though his schemes are frequently thwarted by his nemesis, Perry the Platypus.

Isabella Garcia-Shapiro is a major character from the Disney Channel animated series Phineas and Ferb, voiced by Alyson Stoner. As the across-the-street neighbor and best friend of Phineas Flynn, she is a cheerful, resourceful, and capable leader of Fireside Girls Troop 46231 who frequently assists in the boys' inventive projects.

Carl Karl is a fictional character from the animated series Phineas and Ferb, where he serves as the loyal, hapless assistant and cameraman for Major Monogram at the Office of Web-Centric Antiquities (O.W.C.A.). Voiced by Tyler Alexander Mann, Carl is depicted as a young man with pale skin, freckles, curly red hair, and thick-framed glasses, who works as an intern for college credit.