Justin Puppy's Law is an American animated television series by Maxwell Atoms. The series premiered on October 3, 2020 on Nickelodeon. It revolves around the title character, Milo Murphy, who is a descendant of Edward A. Murphy Jr., the namesake of Murphy's Law, which states that anything that can go wrong will go wrong. The series takes place in the same universe on Cartoon Network as Atoms' previous series Grim and Evil, The Grim Adventures of Billy and Mandy and Evil Con Carne, with several references to the show occurring across season one, culminating in a crossover at the beginning of the second season and continuing throughout.
- Cooper Barnes as Justin Puppy, the title character of the series.
- Kira Kosarin as Sarah Kittyburg, the long-time best friend of Justin's who likes to hang out with him despite his bad luck.
- Dylan Gilmer as Dennis Grizz, as Justin’s friends Melissa Chase and Zack Underwood .. Slater (Mr. Robot) as Elliot Decker, a school crossing guard and Milo’s foe; Williams (Desperate Housewives) as Zack’s mom Eileen Underwood; Chalke (Scrubs) as science teacher Mrs. Murawski; Clement (Flight Of The Conchords) as popular television show character Dr. Zone; Mackenzie Phillips (One Day At A Time) as Principal Milder; Chrissie Fit (Teen Beach Movie) as classmate Amanda Lopez; Martella (Phineas And Ferb) as classmate Bradley Nicholson; Diedrich Bader (The Drew Carey Show) as Milo’s dad Martin Murphy; Pamela Adlon (Californication) as Milo’s mom Brigette Murphy; Kate Micucci (When In Rome) as Milo’s sister Sara Murphy; Dee Bradley Baker (Phineas And Ferb) as Milo’s dog Diogee; Gregory Michael Cipes (Ultimate Spider-Man) as classmate Mort Schaeffer; Michael Culross (Phineas And Ferb) as high school teacher Kyle Drako and Kevin Michael Richardson (The 7D) as Coach Nolan Mitchell.}}</ref> the new kid to the Tri-State Area who becomes best friends with Milo and sticks by Milo even though he is a jinx. He is on the football team. He was once the lead singer of a boy band called "The Lumberzacks". He is often the straight man of the series.
- Diedrich Bader as Marvin Puppy, Justin's father. Just like his son, he has extreme misfortune. He is a safety inspector, mostly due to the fact that when he's around, safety contingencies are pushed to the limit.
- Pamela Adlon as Brigette Puppy, Justin's mother who is an architect.
- Kate Micucci as Tara Puppy, Justin's older sister and fellow Dr. Zone fan who loves her brother despite his recurring bad luck.
- Dee Bradley Baker as Diogee Ex Machina Puppy, Justin's dog. He regularly follows Milo, who keeps telling him to go home as he is almost always not allowed to be wherever Milo is at the moment. His name is pronounced "D-O-G," as if spelling out the word "dog."
Animalopolis County Middle School
- Chrissie Fit as Amanda Leopard, a student in Milo's class who is a perfectionist and Milo’s love interest.
- Vincent Martella as Bradley Newton, a pessimistic and arrogant student who is jealous of the attention Milo gets. Due to the space-time continuum being altered during "The Phineas and Ferb Effect", his right arm is still part plant.
- Greg Cipes as Mort Squirrel, a burly-looking and spiritual student in Milo's class. He is Bradley's best friend, but is friends with Milo and his gang, and is the drummer in their band, Just Getting Started.
- Django Marsh as Chad Van Coff Cow, a classmate of Milo's who is convinced that Mr. Drako is a vampire.
- Alyson Stoner as Lydia Mouse, a classmate of Milo's as well as Amanda's best friend.
- Michael Culross as Kyle Drako Bat, a middle school teacher with a slight Romanian accent. A recurring gag involves him exhibiting traits resembling a vampire.
- Christian Slater as Elliot Decker Deer, the school crossing guard and self-described "Safety Czar". He makes it his mission to protect the city from Milo, to which he constantly hinders Milo and the gang.
- Mackenzie Phillips as Elizabeth Milder Pony, the principal of Jefferson County Middle School.
- Kevin Michael Richardson as Nolan Mitchell Chicken, the football coach of Jefferson County Middle School.
- Sarah Chalke as Mrs. Raccoon, the science teacher of Jefferson County Middle School who holds an unhealthy affection for her desk.
- Laraine Newman as Ms. White Rabbit, another middle school teacher.
Bureau of Time Travel agents
- Dan Povenmire and Jeff "Swampy" Marsh as Vinnie Rat and Balthazar Gator. The pair are time travelers given the assignment to prevent the extinction of the pistachio. They are considered a joke by their fellow time agents because their job is considered inconsequential. Dakota dresses in Disco (1970s) clothing while Cavendish dresses in Victorian era (1870s) formal wear. They initially see Milo as an enemy agent until they learn about Murphy's Law and befriend him in "Missing Milo". Later in the same episode, it is revealed that they were the inspirations for Doctor Zone and the Time Ape. In the second season episode "Snow Way Out", they are revoked from time travel due to Dakota creating unauthorized timelines to save Cavendish. In "Picture Day" they are hired under the orders of Bob Block as janitors at P.I.G (Paranormal Investigation Group). In "Abducting Murphy's Law", they get into an argument that destroys their friendship and ends with Cavendish going rogue from P.I.G.
- Mark Hamill as Mr. Box, Cavendish and Dakota's superior in season 1 and the beginning of season 2.
- Hamill also voices Bob Box, Mr. Box's great-great-great-grandfather in the present day, who hires Cavendish and Dakota as janitors for P.I.G. in "Snow Way Out". He appears throughout the second season.
- Ming-Na Wen and Brett Dalton as Savannah and Brick, colleagues of Dakota and Cavendish. Unlike them, Savannah and Brick resemble traditional secret agents in the vein of James Bond, and drive a time vehicle that resembles a limousine, as opposed to Dakota and Cavendish's, which resembles a Beetle. Their character designs are based on their voice actors, who portrayed secret agents in the television series Agents of S.H.I.E.L.D..
- Vanessa Williams as Eileen Grizz, Dennis' mother who works as a surgeon.
- Phil LaMarr as Marcus Grizz, Dennis' father.
- Adrian Pasdar as Richard Kittysburg, Sarah's father who works as a fireman.
- Jemaine Clement as Doctor Zone, the main character of the eponymous show within a show. He is based on the titular character from Doctor Who, as he travels through time. Orton Mahlson, the creator of The Doctor Zone Files is also the original actor who portrayed the character. Mahlson's name is a parody of writer Martin Olson.
- Sophie Winkleman as Time Ape, Dr. Zone's sidekick.
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