Monitor Monitor(voiced by Jess Harnell)- A robotic monitor lizard who's Doofenshmirtz's main henchlizard. Before the events of the series, he was created by another evil scientist to defeat his nemesis. However, after only two weeks, Monitor Monitor quit because he wasn't treating him with respect; only using him as a last resort or abusing him. Soon after, he met Heinz Doofenshmirtz and they became best friends, right around Heinz Doofenshmirtz noticed Perry hiding behind the mailbox. Although very supportive of Doofenshmirtz's plans, he serves as the straight man. Just like a real monitor lizard, he can swim, climb trees and even use his tail as a weapon. In "Greece Lightning", he thinks Doofenshmirtz is going to replace him, but he(Doofenshmirtz) tells the lizard that he would never replace him. As the series progresses, he becomes closer to Norm and eventually becomes friends with him. He also is very loyal and compassionate towards his boss, and is willing to help him at any time.
In "Nerdy Dancin'" It's revealed that Aloyse von Roddenstein created Monitor Monitor because he wanted prove his superiority over the other scientists.
A running gag in the series is that he gets minorly involved in Phineas and Ferb or Candace's plot depending on the episode(For example, in One Good Scare Ought to Do It!, he accidentally falls out of the plane during Perry and Doofenshmirtz's battle. When Phineas then lands in Isabella's arms, Monitor Monitor lands next to them, and is very happy for them, but runs away to find Doofenshmirtz.
In "Last Day of Summer", he feels depressed after Doofenshmirtz becomes mayor, remembers Vanessa's words, and also decides to also give up evil, to try and help Doofenshmirtz become a better father.
He would also become a main character in Phineas and Ferb: Across the Second Dimension, Phineas and Ferb: Candace Against The Universe, The O.W.C.A Files, Phineas and Ferb: Mission Marvel, Phineas and Ferb: Star Wars, Act Your Age, Phineas and Ferb Save Summer, Summer Belongs to You, Night of The Living Pharmacists and the second season of Milo Murphy's Law. He would appear in almost every episode except for Tales from the Resistance: Back to the 2nd Dimension.