Chespin debuted in Mega-Mega Meowth Madness!, where Ash and his friends found it hiding behind a plant and eating macarons. Clemont tried to befriend it, only for Chespin to stab his hand with its spikes and run off with several macarons in its hands. Later, it witnessed Professor Sycamore being captured by Team Rocket and helped Ash and Clemont track the kidnappers by following a trail of macaron pieces left by Dedenne. Once there, Chespin attempted to steal the macarons Jessie left lying around, only to fail and alert Team Rocket to Ash and Clemont's presence.
After Team Rocket unleashed their Mega Mega Meowth, Chespin was saved from being crushed by the mecha by Clemont, which caused the two to bond a little. Chespin helped Clemont stop the rampaging machine by pulling out its plug, only for Team Rocket to use an alternate power source to continue fighting. Having no choice, Clemont and Chespin battled the mecha, only for all of Chespin's attacks to fail. Even when teamed up with Pikachu, the mecha suffered no damage. However, Blaziken Mask's Blaziken saved the day by weakening the machine with Flamethrower, allowing Chespin and Pikachu to finish it off and send Team Rocket blasting off.
Later, Chespin revealed that it wanted to join Clemont's team as it watched him leave Professor Sycamore's lab. Clemont accepted Chespin's request and was given Chespin's Poké Ball to recall it.
In An Appetite for Battle!, After stealing some of Serena's macarons, Clemont became concerned with how Chespin was out of shape from eating so much. To work off some of the weight, he had Chespin battle Froakie and Dedenne, but it lost to both due to its weight. Later that night, Clemont made a machine to help Chespin out, but Chespin walked off into the woods after smelling some food being cooked. Chespin wandered into Carrie's house and was taken in for the night. Much to its delight, there was plenty of food for it to eat. Later that day, it had a battle with Wylie's Delphox. Chespin barely avoided Will-O-Wisp and was about to be hit by Flamethrower, but Clemont stepped in and blocked the attack. Clemont and Chespin then teamed up to battle Delphox, using its rounded shape to roll and dodge attacks until he lost weight, this allowed Chespin to move much quicker and hit it with a Tackle and Vine Whip. However, Chespin was hit by a powerful Flamethrower, which led to the activation of its ability, Overgrow. Chespin then hit Delphox with one more powerful Vine Whip. It was then about to be hit by Delphox's Mystical Fire, but the battle was stopped due to Wylie experiencing pains in his waist.
Chespin is a gluttonous Pokémon, often going after food, including macarons. As a result, Chespin would keep the food for itself and refuse to give it back. Sometimes, it would eat so much food that it would become heavy, as evidenced in An Appetite for Battle!. Despite its gluttonous traits, it was very brave to go after Team Rocket and respected Clemont after he saved it from getting run over by their mecha. It is also shown to be very determined when it repeatedly tries hitting Team Rocket's mecha despite its efforts being met with failure. Chespin tends to be the more vocal of the group and usually displays a very happy go lucky attitude. In A PokéVision of Things to Come!, Chespin was shown to enjoy the spotlight and was very confident on camera, unlike its Trainer. Also shown to like mimic Ash Hawlucha.
