Marshtomp while it was a Mudkip is a kind and determined Pokémon who is also very protective to his own kind such as the Mudkip babies as seen in A Mudkip Mission. Marshtomp is also shy towards people he has encountered as he runs away quickly. When encountering Brock, Mudkip is initially shy at first. But when he and the baby Mudkip are hanging on the waterfalls, he was convinced by Brock who tries to rescue both of them. Mudkip also demonstrates its alertness when it was teaming up with Brock's Lotad to save all of the Mudkip babies from drifting in a flash flood coming from the waters from the dam after it was broken by James due to being knocked off by his Cacnea's Needle Arm and later showing its seriousness in fighting as he and Lotad use Water Gun to soften the soil from the ground to make Team Rocket's robot to be sink allowing Pikachu to blasts them off with its Thunderbolt.
Throughout the series, Mudkip serves to settle feuds with other Pokémon and often times teaming up with Ash's Pikachu when their feuds are getting too much. This is demonstrated when both are stopping Ash's Corphish and May's Torchic from fighting as seen in A Shroomish Skirmish but failed.
Upon evolving into a Marshtomp, he becomes a well mannered Pokémon who gives a high regard towards romance and a romanticnist similar to Ash's Oshawott whenever Brock sees a girl upon his journey but not like the former which his love disease is worsen after getting rejected. Marshtomp calmly accepts its rejection after seeing other Pokémon are getting along with another Pokémon. In battles, Marshtomp retains his determination in fighting from his pre-evolved form.
Marshtomp debuts as a Mudkip in the episode A Mudkip Mission and it was the only Pokémon to protect the Mudkip babies coming from Swampy. Brock uses Lotad to save the baby but it was saved by Mudkip itself. Brock wanted to befriends it by feeding his Pokémon food but it was interrupted by Ash and the group causing Mudkip to run away. When Team Rocket attempts to steal all the Mudkip babies, Mudkip teams up with Lotad using Water Gun to free them. Mudkip is the only Pokémon to save the last baby from a current until it hangs on the waterfall. Brock eventually saves it from falling.
After the incident, Swampy allows Brock to keep Mudkip due to its liking and has no friends to take care of it. Brock finally received Mudkip to join the group.
Mudkip is used to settle feuds throughout the series. In A Shroomish Skirmish, it teams up with Pikachu to stop both Corphish and Torchic from fighting but failed and Ash's Grovyle is the one to stop then from fighting by separating them with a Leaf Blade. After being separated by a wild Breloom's Mach Punch, Mudkip goes with May to search for the others until it saves Max, Meowth, Pikachu, Torchic and an injured Corphish from Breloom's attack with Water Gun. It also witnesses with Pikachu, Meowth and Corphish to see Torchic angrily evolves into a Combusken to fight Breloom which its leader.
In A Chip Off the Old Brock, Mudkip spars with Ash's Grovyle until it evolves into a Marshtomp as it attempts to hit it with its newly learned Mud Shot but failed and it accidentally hits Mariah's Flaaffy. When trying to save both Mariah and McCauley from Team Rocket, it was used to save them but failed and it accident hits Pikachu and Katie's Mareep forcing them to get cured from their medicine. After the incident, McCauley and Mariah decided to have a new medicine shop leaving Brock and Marshtomp got rejected. In the early events of the episode, May even comments that Marshtomp becomes a Junior version of Brock.
After evolving, Brock's Marshtomp became a strong Pokémon often used to settle feuds or in simple battles. Marshtomp is a well mannered Pokémon which takes after its trainer a little bit in regards to romance. Brock notably used Marshtomp in a Pokémon Contest against May.
- Megumi Hayashibara (Japanese as a Mudkip)
- Shin-ichiro Miki (Japanese as a Marshtomp)
- Lindsey Warner (English 4Kids dubbed as a Mudkip)
- Michele Knotz (English PUSA dubbed as a Mudkip)
- Kayzie Rogers (English as a Marshtomp)
- Brock's Marshtomp is the only Hoenn Starter Pokémon that isn't voiced by Darren Dunstan and later Bill Rogers in its first stage evolution.
- Marshtomp is the only starter Pokémon owned by Brock.
- Marshtomp is the only Hoenn starter Pokémon owned by a main character that didn't reach its final evolution.
- Marshtomp is the first Water-type Starter Pokémon to evolve into its next stage. Second being Ash's Greninja and third being Lana's Primarina.