Mumu first appeared as a Mudkip in Trying to Trounce Torchic, as one of the two Pokémon that was in Professor Birch's bag. When Ruby was rescued by Sapphire from a pack of wild Mightyena, she took him to her Secret Base. He thought she was a Pokémon of some sort, and was shocked to the spine when she started talking. The two ended up getting into a fight after he ordered Nana to Bite the supposed "Pokémon." Still holding his own Pokémon from fighting back, he phoned Professor Birch on the Pokégear that was in his bag, and was told to choose the "blue Pokémon," which he then chose over the "green one". However, against Sapphire's well-trained Chic, he was no match, even with his type advantage. He later nicknamed him "Mumu" after he defeated a wild Dustox and decided he would be best for the Tough category of Pokémon Contests.
In Tripped Up by Torkoal, Mumu evolved into Marshtomp, gaining the awe of the Pokémon Fan Club Chairman, who also adored Nana and Kiki. He tried to help Ruby enlist in the Contests there, but with no Hoenn Contest Pass, he wasn't accepted. Instead, the two ended up being dragged into a mire when three Magma Admins kidnapped the two along with Captain Stern. When Ruby awakened Mumu popped out of his Poké Ball to help free him, but to no avail. Upon realizing that the one interrogating Stern for information now was in cahoots with Tabitha and Courtney, who had attacked him and Sapphire before, Ruby used his Pokéblock-making skills to lure Blaise's Slugma into the Submarine they were planning to steal. Mumu managed to counter Slugma's fire attacks, but soon fell to Blaise's illusions. After overcoming it, he attacked with Earthquake, allowing the two to reach the escape pod.
In Hanging Around With Slaking I, Mumu battled Ruby's father, Norman. After Nana and Kiki were defeated due to Norman knowing how they attacked it was revealed that they were a distraction for Mumu to set up a Mud Shot attack, which landed a direct hit after shooting through some pipes. Norman quickly counters with Vigoroth and it seems the two were evenly matched. Ruby soon attempts to fire Mumu's high-powered Mud Shot, but it is stopped by Mr. Stone's Castform's Sunny Day.
In It's Absol-utely a Bad Omen, Mumu advanced Ruby through the Normal Rank of the Tough Contest. In Going to Eleven with Loudred & Exploud I, Mumu is used alongside Ruby's other Pokémon to battle against Courtney and her Ninetales. Although Ruby had a five-to-one advantage, his team was beaten due to Ninetales having control of nine different fireballs.
In I More Than Like You, Luvdisc I, Mumu advanced Ruby through the Super Rank of the Tough Contest. After the contest Mumu is used in Ruby's battle against Wallace alongside the rest of his team, bar Feefee, and loses.
In Master Class with Masquerain, Mumu evolved into Swampert. Soon after Mumu advanced Ruby through the Hyper Rank of the Tough Contest.
In The Beginning of the End with Kyogre & Groudon X, Mumu, alongside Ruby and Sapphire's other Pokémon, battled Archie and Maxie. He and Chic attempted to deliver a decisive blow, but failed to do any real damage. In The Beginning of the End with Kyogre & Groudon XIV, Mumu and Chic attack the meteor fragment coming from Archie and Maxie, which sends a powerful blast of energy out. The energy hits Maxie and Archie's foreheads, turning them back to normal and stops the power of the Red and Blue Orbs.
In It All Ends Now VIII, Mumu battled alongside the rest of Ruby and Sapphire's Pokémon against Maxie and Archie. Later that day, Mumu is shown to have won the Master Rank of the Tough Contest, wearing a Yellow Scarf.
In Cunning Kirlia, Mumu advanced Ruby through the Battle Dome. Despite being one of his three options to fight against Emerald he was not used.
In Facing Gulpin Is Hard to Swallow, Mumu helped Ruby, Sapphire, Emerald, and Noland advance through the Battle Tower to reach Guile Hideout.
In The Final Battle III, Mumu is taught Hydro Cannon. He was soon used alongside Blasty and Silver's Feraligatr to attack with Hydro Cannon to help destroy Guile Hideout's fake Kyogre.
In Omega Alpha Adventure 3, Mumu, alongside Chic and Sceptile, trained under Ultima in order to fully master the ultimate moves. The training was issued by Steven Stone and Mr. Stone as part of a plan dedicated to preventing a giant meteoroid from crashing into the planet. With Ruby having business elsewhere, Steven trained Mumu in his Trainer's stead. When Ruby returned in Omega Alpha Adventure 8 to stop the attack Zinnia lead on the Sea Mauville, he Mega Evolved Mumu for the first time and used him to battle Zinnia's Goodra.