Croagunk was first seen resting outside a supermarket in Gymbaliar!. When Meowth emerged from it, Croagunk followed him to where Team Rocket was using a fake Gym to steal unsuspecting Trainers' Pokémon. Their plan was going badly as it required Jessie to win, but her Dustox kept losing, so she decided to use Croagunk and soon won many battles.
After Ash and his friends entered the Gym, Croagunk lost against Ash's Aipom. Team Rocket resorted to grabbing Ash's Pikachu and running off, leaving Croagunk behind in the process. Brock was the first to notice Croagunk, and felt a connection, Croagunk later willingly went into Brock's Poké Ball.
Like Brock's Bonsly, Croagunk has attacked Brock in order to continue the series' running gag of keeping Brock away from the girls who are the objects of his affection, to the point that even Croagunk's presence can be sensed by Brock, which may make Brock suddenly halt his romantic advances. When forcefully dragging Brock away from girls, he usually uses Poison Jab, which usually makes Brock suddenly collapse so Croagunk can easily drag Brock away. However, when Brock pledged to save Oralie's Magikarp and Haley's Feebas in Ya See We Want An Evolution!, Croagunk jabbed Brock, which caused Dawn to remark that Croagunk may have gone too far. Croagunk generally chuckles or laughs after attacking Brock, so it is possible that he actually enjoys attacking Brock. However, Croagunk has also protected Brock from harm, such as falling off a bridge in Bibarel Gnaws Best! and from Saturn's Toxicroak's Dark Pulse attack in Enter Galactic!.
He has some unique characterization, as he acted alone many times. His actions were sometimes part of an episode plot. He has a unique fighting style, as shown in Leave It To Brocko!, by defeating Seviper by repeatedly using Brick Break while Seviper was using Wrap on him, eventually ending in a draw.
Croagunk has been shown to have his own pace for things like eating, attacking, or following Brock. Similar to Ash's Treecko, he is also a bit of a loner, preferring to sit alone when eating or just resting. Although he is seen mostly angry or without any emotion, he was seen happy in Doc Brock!. In An Angry Combeenation!, he didn't like the sweet taste of Honey, which may imply his nature is brave, quiet, sassy or relaxed.
Croagunk showing off his behavior
He is a very powerful Pokémon, as demonstrated in his debut episode where he beat many Trainers while he was under Jessie's command. He has since become Brock's main battling Pokémon and has proved his power again and again, even stopping a Draco Meteor attack in The Rise of Darkrai. He seems to have a rivalry developing with one of the other strength-oriented Pokémon on the cast, Ash's Buizel. He also has an extreme rivalry with Saturn's Toxicroak, to the point of actually ignoring Brock's advances on women. In several episodes he has been shown to be unusually observant, and intelligent.
He has good hearing, as he could hear Buneary in the bush in Setting the World on its Buneary! and Team Rocket sneaking into Oreburgh Gym in O'er the Rampardos we Watched!. He is sometimes aware of many strange or some unusual situations; in Malice in Wonderland!, he saves the day when he jabs Brock and saves him from Mismagius's illusions. He later was able to sense the oncoming Team Galactic attack in Losing Its Lustrous! — in particular, the approach of Saturn's Toxicroak.
Croagunk pulling Brock away after jabbing
He also can do a remarkable impression of a Politoed, as seen in All Dressed Up With Somewhere To Go!, where his convincing cosplay and acting won the Pokémon Dress-Up Contest and the Pokémon Egg prize.
Croagunk has a fascination at looking at his reflection. This can be seen in several episodes, such as Doc Brock! and Hold the Phione!.
In A Faux Oak Finish!, Croagunk guided the gang through a marsh-like area, he then encountered a wild Politoed and battled it.
In The Battle Finale of Legend!, Croagunk's rivalry with Saturn's Toxicroak was settled when he defeated Toxicroak with a Brick Break on its head.
Croagunk reappeared in SS027, once again halting Brock's romantic advances on Nurse Joy. Witnessing the dynamic between Brock and Croagunk, Cilan commented on their unique and spicy relationship.