Waddles' official series debut is in "The Time Traveler's Pig", where Mabel wins him in a "Guess the Critter's Weight" game which is run by a carny. She looks at the animals, until one pig oinks, which she interprets as either "Mabel" or maybe "Doorbell". Taking this as a sign, Mabel asks the carny to let her guess the pig's weight, and he tells her its name, "15-Poundie". Mabel guesses correctly that it weighs 15 pounds. The carny accuses Mabel of being a witch after giving her the pig. She takes the pig, renames it Waddles, and quickly forms a bond with it.
The bond between Mabel and Waddles is strengthened each time the twins travel through time, as they do things such as sharing pizza, and taking photographs in photo booths. This results into her spiraling into depression when she loses him to Pacifica Northwest in the only successful Dipper timeline. After realizing how much Waddles means to her, Dipper goes back in time and sacrifices his chance with Wendy, allowing Mabel to re-win Waddles. To show her appreciation, Mabel sends Waddles to retrieve the caramel apple Robbie is sharing with his new girlfriend, Wendy. This startles Robbie, causing him to drop the apple and knock over a barrel of hot water, painfully shrinking his pants.
In the next episode, "Fight Fighters," Waddles sits next to Mabel as she plays a card game with Grunkle Stan, and before the fight asks if they want her to send Waddles on Robbie again, referring to the previous episode. He is also there when Mabel watches Grunkle Stan while he watches TV. At another point he sits next to Mabel at the table while Dipper worriedly paces about his upcoming fight with Robbie.
In "Summerween," Waddles stays at the Mystery Shack instead of going trick-or-treating with Mabel, Candy Chiu and Grenda. His Summerween costume is a business suit. Grunkle Stan later uses Waddles in attempting to scare off two kids, and made it look like Waddles was coming out of his stomach. At the end of the episode, Waddles eats the bag of candy Stan received while everyone else watches a horror movie for the remainder of the night. Grenda, Candy and Mabel are later seen taking pictures of Waddles and adding captions, similar to internet memes such as the "lolcat".
In "Boss Mabel," Waddles is first seen watching Cash Wheel with Dipper, Mabel and Grunkle Stan while eating Chipackerz. When Mabel becomes the boss of the Mystery Shack, she makes Waddles her secretary. Near the end of the episode, Mabel asks Waddles for his opinion on Stan's apology song. Waddles reets, and Mabel declares "Take thirty."
In "Bottomless Pit!", Waddles is first seen in Dipper's story, "Voice Over." Mabel, Wendy and Soos play a variant of spin the bottle called "Spin the Pig," in which they spin Waddles and whomever he lands on must kiss him. He is later seen in Mabel's story, "Trooth Ache." when Mabel asked Waddles for advice, and then pretends like Waddles is answering.
In "Carpet Diem," Waddles is shown squirming on the carpet and when Soos touches him, they switch bodies. When this happens, Soos leaves the Mystery Shack on an "adventure" while Waddles walks around the Mystery Shack looking for things to eat. Eventually Waddles ends up in the gift shop, and starts chewing on some clothes. When Wendy walks in and sees Waddles (whom she mistakes as Soos obviously since Waddles is in Soos' body) she leaves, and next Grunkle Stan walks in. After explaining that Stan needs to cut Soos' pay, Waddles puts his hand on Stan's nose and starts disturbing him. Eventually Stan gives in, and gives Soos a raise instead. Later in the episode, Soos and Waddles get back into their own bodies.
Waddles is seen briefly in "Boyz Crazy" when he is sitting by Candy while the girls are getting ready to go to the Sev'ral Timez concert. He is later seen when Deep Chris pets him on the head and says, "What up, girl?"
When Li'l Gideon takes over the Mystery Shack in "Gideon Rises", he takes possession of Waddles and makes him the mascot for Gideonland, renaming him Li'l Gideon Jr. After Gideon's defeat, Waddles is reunited with Mabel, who burns his Li'l Gideon Jr. costume.
Perhaps Waddles' most prominent role is in the "Abaconings" segment of "Little Gift Shop of Horrors", in which he accidentally eats brain-enhancing jelly derived from a percepshroom. The jelly makes the pig so intelligent that he builds a machine that he uses to drive around and communicate with. As a genius, he develops a bond with Dipper, but comes to miss all the fun he has with Mabel and eventually reverts himself back to his original intelligence level.
When Mabel and Waddles are trapped within Bill Cipher's prison bubble in "Weirdmageddon", Waddles becomes a giant who serves as transportation in Mabeland. Later, Waddles assists Mabel, Dipper, Wendy and Soos in escaping back to the real world, reverting back to his original size in the process. After Bill's defeat, when the family tries to recover Stan's lost memory, Waddles manages to kick-start it by jumping onto his lap and licking his face, which Stan instinctively responds to by telling them to get Waddles off of him. At the very end of the episode, Mabel says that her and Dipper's parents won't allow her to keep Waddles, but she can't bring herself to say goodbye to him. Stan and Ford then make the bus driver allow Waddles to come on to the bus with Dipper and Mabel.
Waddles is a light pink pig with a light brown/tan dot on his left eye, with light brown/tan spots on the the left side of his back. His snout is light pink, he has a black curly tail, his hooves are brown/black and he has a rather plump face. Waddles also weighs 15 pounds, and his favorite food is caramel apples. On Summerween, Waddles wears a costume which looks like a business suit, a black coat, a dark brown collar, a white shirt and red tie, and black shoes.
- Waddles was named by Aury Wallington, who is one of the writers for the show. She named him after her own pet pigs that she owned during her childhood. Every year she'd raise, care, and love it for months, and every year her family would cook and eat Waddles. She'd get another pig for 4H and the cycle repeated itself for years.
- Alex Hirsch gave Mabel a pig because his sister, Ariel, always dreamed of having one and also has a pig shrine in her room.
- It is not shown exactly where Waddles came from before the contest where Mabel got him; possibly a farm.
- Dee Bradley Baker, who voices Waddles, also voices Perry the Platypus and Pinky the Chihuahua.