Uncle Grandpa

Main characters from the series; from left to right: Uncle Grandpa and Belly Bag, Pizza Steve, Mr. Gus and Giant Realistic Flying Tiger (bottom)

• Uncle Grandpa (voiced by Peter Browngardt, Pendleton Ward in "For Pete! Love, Pen"), a comically dumb character yet also strong, magical, and shapeshifting and he loves helping kids with their problems.
• Mr. Gus (voiced by Kevin Michael Richardson), a calm collected dinosaur who usually serves as the voice of reason.
• Pizza Steve (voiced by Adam DeVine, Pendleton Ward in "For Pete! Love, Pen"), an anthropomorphic prideful pepperoni pizza slice with sunglasses and a member of Uncle Grandpa's crew of which Uncle Grandpa idolizes.
• Giant Realistic Flying Tiger, a static photographic cutout of a tiger that Uncle Grandpa rides on to get around.
• Belly Bag (voiced by Eric Bauza), Uncle Grandpa's talking red magic fanny pack and best friend.

• Tiny Miracle the Robot Boy (voiced by Tom Kenny), Uncle Grandpa's hand built sergeant that does tasks around the van.
• Beary Nice (voiced by Audie Harrison) and Hot Dog Person (voiced by Eric Bauza) – These two are the stars of their own recurring segment, "New Experiences".
• Charlie Burgers the Ball-Loving Dog Next Door (voiced by Brian Posehn), a talking dog that befriends Uncle Grandpa and his friends.
• Santa Claus (voiced by Bob Joles) is the famous figure for Christmas and is revealed to be Uncle Grandpa's brother in the Christmas special. They have had a troubled relationship since 1983.
• Frankenstein (voiced by Mark Hamill), a tag-along with Uncle Grandpa in his adventures.
• Xarna, She-Warrior of the Apocalypse (voiced by Eric Bauza) is a cyborg warrior. She is on a mission to get gas for her Mad Motorcycle.
• Ule Gapa (voiced by Tom Kenny), an unfriendly blue big mouthed alien with 4 sharp teeth who is constantly yelling at people. He appears first in an intermission when Uncle Grandpa tells Mr. Gus that his own logo is an imposter and destroys it using a magical spells, and he comes out saying “Hey! YOU’RE NOT ULE GAPA, I’M ULE GAPA!”
• Evil Wizard (voiced by Rob Schrab), a wizard who goes around trying to make everyone's day horrible by humiliating them but he actually makes their lives better and his worse.
• Priscilla Jones (voiced by Lena Headey), Uncle Grandpa's nemesis with a British accent and the main antagonist of the series under the pseudonym “Aunt Grandma”. She wants revenge on him for ruining her science project and to usurp him as Aunt Grandma.
• Crispin Mulcahy (voiced by Michael Dorn), a miniature strongman who gets preserved in gold and becomes the RV's lamp. He later reports on the RV presidential election in "Uncle Grandpa Runs for President".

• Belly Kid (voiced by Zachary Gordon), a kid who has a big belly. He was first ashamed of it, but Uncle Grandpa taught him the best features of having a big belly. He appeared in "Belly Bros".
• Caleb (voiced by Jonathan Adams), a boy that Uncle Grandpa takes out of a math test to go battle Evil Wizard in outer space. He appeared in "Tiger Trails".
• Melvin (voiced by Jarid Root), a bossy kid who likes to play Space Emperor, with him being the emperor. When Uncle Grandpa accidentally sent him into another dimension, he was sent to a planet where he's the emperor. He discovered what it's like being a servant, and learns his lesson after Emperor Krell bosses him around. He loves dinner sandwiches, and hates mayonnaise. He appeared in "Space Emperor". His catchphrase is "I decide who does what!"
• Melvin's Babysitter (voiced by Grey DeLisle-Griffin), an attractive teenage girl with braces who babysits Melvin in the episode "Space Emperor".
• Eric (voiced by Eric Bauza), a kid who doesn't have a nickname, so Uncle Grandpa helps him be legendary to get his nickname. In the process of becoming legendary he becomes tall and muscular. In the end he gets the nickname, "Cupcake", because he loves cupcakes. He appears in "Nickname".
• Mary (voiced by Pamela Adlon), a nervous girl. She takes her driver's test, but fails. Uncle Grandpa takes her on a test and helps her pass. Uncle Grandpa gives her her own "Freedom and Independence USA" truck for passing her test. She appears in "Driver's Test".
• Dennis (voiced by Tom Kenny), a kid who wanted to finally pass his teacher's hard class and avoid going to summer school, until Uncle Grandpa comes and eats his homework and putting Dennis in danger of going to summer school. So Dennis and Uncle Grandpa go to Egypt to get a real pyramid to make sure Dennis gets an A+++++-+ on his project. Dennis appears in "Uncle Grandpa Ate My Homework!".
• Mrs. Numty (voiced by Grey DeLisle-Griffin), Dennis' teacher. She appears in "Uncle Grandpa Ate My Homework!" and "1992 Called"'.
• Guillermo (voiced by Eric Bauza) – Guillermo is a kid who had an awesome new bike until the Perpetual Persistence crushed it. In Uncle Grandpa and Belly Bag's absence, Mr. Gus and Pizza Steve help Guillermo by giving him a trashcan and saying it was a "Magical Uncle Grandpa Bike". Guillermo appears in "Uncle Grandpa for a Day".
• Susie (voiced by Grey DeLisle-Griffin), a little girl who was afraid of the dark, but learned how to combat her fears by imagining herself as a scary monster. She appears in "Afraid of the Dark".
• Adam (voiced by Dee Bradley Baker) – Adam is a kid who couldn't become the master of a very hard video game, until Uncle Grandpa and Pizza Steve shrink to get inside his brain to become better at video games. He would always have his eyes barely open showing that he's been playing videogames too much, and his eyes are sore from playing too much. Adam appears in "Brain Game".
• Angry Man Johnson (voiced by Roger Craig Smith) – Angry Man Johnson is a grumpy old man who hates Charlie Burgers. Judging by his name, he is always angry at everyone, including Uncle Grandpa. Angry Man Johnson appears in "Charlie Burgers".
• Austin (voiced by Carlos Alazraqui) – Austin is a kid who had a lot of imperfections so he asked Uncle Grandpa to turn him into a robot to become the most perfect kid, and to eliminate all imperfections. Austin appears in "Perfect Kid".
• Shaquille O'Neal (voiced by himself) – The former professional basketball player who is an old friend of Uncle Grandpa. In the past, Uncle Grandpa helped Shaq realize his dream of being a stand-up comedian. He appears in "Perfect Kid" after Uncle Grandpa is chased into a comedy club that Shaq was performing at, where he helps Uncle Grandpa fight against Austin 2.0 by combining with the audience in a manner similar to a giant robot. Uncle Grandpa finds the jokes Shaq makes to be very funny.
• Akira (voiced by Jessika Van), a Japanese kid who wanted to make the best action packed monster movie of all time. Akira appears in "Big in Japan".
• Riley (voiced by Scott Menville), a teenage boy who fails a test, his father grounds him and demands him to fold laundry all weekend instead of going to a party.
• Josie (voiced by Grey DeLisle-Griffin) – Josie is a girl who was trying to make duck lips for a social media site and tries to get a lot of followers and likes. Josie appears in "Duck Lips".
• Emily (voiced by Susanne Blakeslee) - Emily is a girl scout.
• Isabella (voiced by Nika Futterman) - Isabella is a girl who is an aspiring inventor but her inventions are useless. Isabella appears in "Inventor Mentor".

• Ham Sandwich Jones (voiced by Steven Blum) – A rotund nerdy teenager who started out hating Uncle Grandpa, but later grew to like him. In the Secret Mountain Fort Awesome episode "5 Disgustoids and a Baby", he appeared as less responsive, and more stingy while intensely playing a portable gaming system. He can be seen in the show's intro and has a very brief role in "Big Trouble for Tiny Miracle".
• Little Judy Jones (voiced by Grey DeLisle-Griffin) – Ham Sandwich's fat mother.
• Remo (voiced by Tom Kenny) – A destructive popular kid, who similarly started off hating Uncle Grandpa, but ends up warming up to his antics. He appears in Secret Mountain Fort Awesome episode "Secret Mountain Uncle Grandpa".
• Remo's Friends (voiced by Steve Little and Tom Kenny).
  • Kev (voiced by Jon Heder) – Kev is a destructive teenager who thought art was dumb until Uncle Grandpa convinced him that art is pretty fun. Kev appears in "Viewer Special". The character originally appeared in a live-action film starring Browngardt called The Last American.[8]
• Crazy Driving Man (voiced by Paul Rugg, Wallace Shawn), a man who wears framed glasses, who is the somewhat "uncool" father of his son Remo. He reappears as a driving instructor in Uncle Grandpa's 7th episode, "Driver's Test", and claims to be the father of a crazy driving baby from another universe and voiced by Wallace Shawn.

The first-season episode "Christmas Special" was aired in December 2014. A double-length Christmas special, it centers on Uncle Grandpa who reluctantly agrees to disguise himself as his brother Santa Claus, after the latter character injures a leg. The episode received mostly praise in international publications,[9][10][11] and in the United States it was viewed by 1.5 million.[12] Cartoon Network developed a browser game adaptation named Sneakin' Santa to promote the episode.[13] The episode was well received by the critics during its respective Christmas season.

A crossover special between Uncle Grandpa and Steven Universe was aired as an episode of the latter series' second season in April 2015.[14] Conceived of by the creator of the latter series, Rebecca Sugar,[15] the episode has Uncle Grandpa teach Steven how to bear a shield from his gemstone. Nearly two million viewed the episode,[16] which received acclaim from critics.[17][18]

The second-season episode "Guest Directed Shorts" was aired in June 2015 and consists of three animated shorts. The first short, directed by M. Wartella, has Uncle Grandpa using time travel in order to find the best hamburger; the second, by Pendleton Ward, has Pizza Steve beatboxing with Uncle Grandpa in the park; and the third, by Max Winston, has the RV gang getting trapped in Uncle Grandpa's claymation mind after their television set breaks down. Wartella is known for his work on the animated series Superjail! and Mad, while Ward is the creator of Adventure Time; Winston is a professional stop-motion artist.[19][20] The episode was viewed by 1.3 million.[21] The episode was very well received by the critics, while Winston's short was nominated for best television production at the 2015 edition of the Annecy Film Festival.[22]

Uncle Grandpa Babies first appeared as a short in the 2014 episode "Grounded" as a faux Cartoon Network ad that presents a series called "Uncle Grandpa Babies" and claims to be made by the same creators of Adventure Time (Pendleton Ward) and Steven Universe (Rebecca Sugar). One year later, a full episode based on the babies saving America from a foreign country's missile launch aired on August 20, 2015.

In light of the special premiering, reruns of Baby Looney Tunes returned to Cartoon Network.