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 against 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.
Mumu was noted in his debut appearance for being less coordinated, swift, and graceful compared to the rest of Ruby's team at the time, and habitually left a trail of mucus from his nose. Despite this, Mumu quickly won Ruby over with his resourcefulness and ability to use his fin's enhanced senses to detect enemies. As Ruby's Pokémon Contest representative for the Toughness category, Mumu exhibits great fortitude and power in contests and battle, becoming Ruby's main powerhouse as the chapter developed. After evolving, many of Mumu's clumsier traits as a Mudkip seem to have faded.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
Although Mumu doesn't evolve into his final stage until Volume 19, the cover art depicts him as a Swampert along with Sapphire's Chic (which by then was already a Blaziken) the volume prior. This might be due to the fact that before being released in volume form, few to several rounds, even within the same chapter, may be left uncollected, and so the fans were expected to be aware of his evolution already, which happened mere chapters after the end of this volume. Alternatively, it may just be a result of the fact that Mumu and Chic were always seen in the same evolution stage when placed in a volume cover together, just as Exbo, Feraligatr, and Megaree were in the covers for the Gold, Silver & Crystal chapter.
Mumu's Contest call is, "Regardless of the effects of time, the toughness that this body encompasses never changes. Boiling with raging Torrent, this is the Pokémon Swampert—Mumu!"
Mumu is the only male Pokémon that Ruby currently owns.