Brain is the tritagonist in the Inspector Gadget franchise. He is Penny's dog who is the only one that knows Penny is the one who solves Gadget's missions.
Brain typically has to follow Gadget to wherever he is going on his missions and has to disguise himself as a civilian to prevent Gadget from noticing him. Only to find out that Gadget thinks he's a M.A.D agent or another type of person, even though it's obviously him.
Brain always stays close to Gadget to keep him safe during his missions and usually saves him from being eliminated by M.A.D. agents. Brain is a master of disguise and disguises himself to keep being recognized by Gadget. Gadget often mistakes Brain as a M.A.D. agent, often chasing him and trying to arrest Brain. Despite Brain's quick wits and last-minute rescues, he would have also failed to solve the mission assigned to Gadget if Penny wasn't secretly separated to help solve the case as well. This is the story line for nearly all the episodes. However, all of M.A.D. agents' attempts to kill Gadget were not all entirely foiled by Brain; in every episode, Gadget managed to escape death by pure luck a few times or by a rare moment of competence and usefulness.
Brain has yellow fur, white patches on the tips of his ears, and a red nose. He wears a red collar that contains a retractable microphone and speaker to keep in contact with Penny. Penny quickly understands what Brain says, even though he only makes dog-like sounds and never actually speaks. He spoke in the Gadgetinis episode, No Brainer, due to Penny building him a translation collar.
In the CGI version, he tends to stand upright, his fur is more orange and the tips of his ears are a light orange. His collar now projects video calls from Penny.
- He is absent in Gadget and the Gadgetinis. He only appears in "No Brainer," but it turns out he has developed a strong fear of gadgets due to his hatred of the times when he was forced to rescue Gadget at every turn and Gadget mistaking him for MAD Agents. He was relieved to never have to defend Gadget again and he even put up signs around his home that says he doesn't want Gadget around, so Penny lied that Brain was dead. Brain still loves Penny as they were best friends in the original cartoon.
- Though he is absent in Gadget and the Gadgetinis, he still appears in the French closing credits.
- In the live-action film, he is portrayed as a beagle and shows no personification, despite being a yellow dog in the cartoon who can walk on his hind legs and often dressed up in disguises. At the end of the film, he has a line of dialogue spoken by Don Adams, the voice actor for Gadget in the original cartoon.
- In Inspector Gadget 2, he plays a slightly larger role when Gadget uses Baxter's bark translator to enable Brain to tell him the whereabouts of Penny after she is kidnapped by Dr. Claw. Here, Brain was voiced by Jeff Bennett.
- In Polish, his name is Łepek.
- His, Penny Brown, and Inspector Gadget's appearances in the Stephen Squirrelsky and Friends' movie spoof travels are Simba (Shrek), Movie-Spoof Villains' Revenge, The Great Hedgehog Detective (FamousImagination50's Style), The Sword in the Stone (Stephen Druschke's Animal Style), Vanellope Von Schweetz (a.k.a. Pippi Longstocking), The Train and the Boat, Toon Age (J.B. Eagle's Style), Hoodwinked! (Dalmatian Tunes' Style), Beauty and the Grizzly Bear, The Rescuers (TrainBoy43's Style), Nikki's Adventures of Sing Along Songs Episode 4, The Rescuers Down Under (TrainBoy43's Style), The Secret of NIMH (Nikkdisneylover8390's Human Style), Nikki's Adventures of Sing Along Songs Episode 6, The Little Mer-Lioness II: Return to the Sea, Skunkules, Home on the Range (DalMatian Tunes' Style), Kermit's Winter Wonderland, The Little Jungle Boy, Kermiladdin 2: The Return of McLeach, AiAi the Monkey and A Day for George, The Reindeer and the Deer, Animal Story 2 (Stephen Druschke's Style), AiAi's Grand Adventure: The Search for Tails, A Cartoon Character's Life, Nikki's Adventures of Sing Along Songs Episode 7, Here Comes Winnie the Pooh, Animals, Inc. (Dragon Rockz's Style), Simba 2, Miss Bianca and the Bernard 2: Fievel's Adventure, FernGully: The Last Rainforest (Dragon Rockz's Style), A Pooh in Central Park, Bubblesrella 2: Dreams Come True, The Brave Little Piglet, Kung Fu Owen, Nikki's Adventures of Sing Along Songs Episode 8, The Thomas O'Malley Movie, Kermit Returns, Fievel the Red Nosed Mouse and The Island of Misfits, Nikki's Adventures of Sing Along Songs Episode 9, A Cartoon Character's Life Activity Center, Cloudy with a Chance of Meatballs (Justin Quintanilla's Style), Finding Alvin (J.B. Eagle's Style), Animated Tale (TheTure DisneyKing's Style), The Forest Book 2 (TrainBoy43's Style), Nikki's Adventures of Sing Along Songs Episode 10, Madagascar (FamousImagination50's Style), Sleeping Beauty (Justin Quintanilla's Style), Aurorastasia, Despicable Me (Justin Quintanilla's Style), We're Back!: A Jungle Animal's Story, Jimmynocchio, and The Wizard of Oz (Uranimated18's Style) and will even be in more movie spoof travels.