Brock obtained Marshtomp as a Mudkip in A Mudkip Mission. Mudkip was a wild Pokémon, though it frequented Old Man Swamp's property on Dewford Island, where he raised Mudkip for the Pokémon League. It tended to act as a guardian to the young Mudkip and was noticeably larger. Old Man Swamp feared that Mudkip would eventually become lonely once all of the hatchlings were sent out to the new Trainers. Brock eventually befriended it after a few failed attempts. Later, Mudkip, Brock, and Brock's other Pokémon, Forretress and Lotad, had to rescue a baby Mudkip from falling over a waterfall. They managed to save it, but right after the ordeal they were attacked by Team Rocket. Mudkip and Brock's Lotad used Water Gun on the ground around the mecha to make it sink into the ground, and Ash's Pikachu finished the job with Thunderbolt. Old Man Swamp later suggested that Mudkip go with Brock, to which it happily agreed.
During the group's journey through Hoenn, Mudkip was usually seen alongside Brock's Lombre. The two were often called out to use a dual-Water Gun attack.
In Turning Over a Nuzleaf!, Mudkip, along with Pikachu, Treecko, Corphish, Torchic, Silcoon, and Lotad, were separated from the group after a Loudred attacked them. Mudkip and Pikachu attempted to lead the group of Pokémon as they searched for their Trainers. However, a group of Nuzleaf began to capture the Pokémon one by one after Corphish damaged the leaf on one of the Nuzleaf's heads. Mudkip and Pikachu were not captured and worked to save the others until Ash and the group arrived.
In Judgment Day!, Mudkip was used in a mock battle alongside Lombre against Ash's Swellow and Corphish to help Jimmy train as a referee. Lombre was quickly pushed into a tough position and Brock had it play dead to give Jimmy an easy judgement. As Jimmy wondered what to do, Ash commanded Swellow to attack Lombre, who retaliated with Razor Leaf. The attack failed to knock out Swellow and it and Mudkip attacked with dual Water Guns, which Corphish blocked. Jimmy declared Corphish unable to battle, though it sprung back up immediately after. His teacher scolded him for the rash judgement and the battle was put to an end.
Initially, Mudkip was distrustful of strangers and was very protective of those it cared for, similar to Ash's Bulbasaur. This is shown in its debut appearance, where it viewed Brock and the others as intruders of its land. Once Brock gained its trust, it turned into an extremely loyal Pokémon and felt safer around strangers. During its travels with Brock, it was shown to be more of a sensible type and it was seen assisting Pikachu in breaking up and resolving arguments between the other Pokémon. It is also friendly with younger Pokémon. It was particularly useful when it came to locating objects and missing people or Pokémon thanks to the fin on its head, which was hypersensitive to any vibration.
When Mudkip evolved into Marshtomp, it retained the same personality but also developed somewhat of a flirtatious side, like its Trainer, as seen in A Chip Off the Old Brock when it fell madly in love with Mariah's Flaaffy.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
In the TCG
Brock's Marshtomp was featured in the TCG multiple times. The following is a list of cards named either Brock's Mudkip or Brock's Marshtomp.
Brock's Marshtomp Cards listed with a blue background are only legal to use in the current Expanded format. Cards listed with a silver background are legal to use in both the current Standard and Expanded formats.