Chespin was given by Professor Sycamore to Mairin as her starter Pokémon and she nicknamed it Chespie. Chespie is almost always out of its Poké Ball, walking with Mairin and making sure she stays out of trouble - or, at least, help her out. Alain also helped Mairin to catch her Bébé by using her Chespie. Chespie used Vine Whip, Pin Missile and Toxic to counter Bébé's Magical Leaf and to defeat it, allowing Mairin to catch it. Chespie accompanied Mairin to Hoenn and was worried when its mistress got stuck in a bush.
After Alain and Steven flew off to find the source of megalith energy, Mairin and Chespie sneak onto the airplane. The airplane was following Primal Groudon, so Mairin asked Chespie to use Ice Beam, only to be reminded Chespie cannot learn that move. Seeing Alain was in danger, Mairin and Chespie went to save him, the latter using Pin Missile to crush the icy boulders made by the clash of Primal Groudon and Primal Kyogre. After the battle was over, Chespie high-fived Steven's Metagross, who helped protect them.
Chespie also accompanied Mairin to see Alain and his Charizard again when they went back to Kalos without saying it to Mairin and Chespie. They visited Lysandre's lab and saw Alain and his Charizard battling several trainers with their Mega Pokémon. After a couple of battles, Mairin and Chespie went to Alain and Mairin asked why Alain didn't want to go on their journey anymore. When Alain didn't give a proper answer, Chespie became angry but Mairin grabbed it before Chespie could do something. Alain then said that he couldn't get stronger if she is around, leaving Mairin heartbroken. Mairin then sat on the stairs and asked Chespie if it could leave her alone. Chespie wandered off in Lysandre's lab and eventually found a hidden labolatory where several scientists were doing experiments. The being called out to Chespie and the experiment failed. Chespie started to glow with a strange green aura and protected the being from the scientists. Chespie used Vine Whip to let the being escape through the air shaft and Chespie got exhausted and sat down, still glowing from the strange aura. Mairin later found Chespie and brought it to Nurse Joy to heal it. Nurse Joy stated that Chespie wasn't in critical condition but she didn't know what the cause was that Chespie was in coma. Lysandre came and said that he and his lab would take care of Chespie while he is in coma. Mairin was sitting beside Chespie while Chespie was still in coma after it was brought to Lysandre's lab.
Chespie was still seen in comatose with Mairin at its side when Alain asked Lysandre how Mairin and Chespie were doing. Chespie was later shown in a video message where Mairin told Alain that one of Chespie's spikes moved the other day. Mairin spoke to the heroes and Alain was talking to Sycamore, as they both remembered Chespie, who was with Mairin during her adventures. Mairin also told Chespie was still in coma and wished for her to recover.
When Mairin learned that Lysandre had no intention of helping Chespie get better once his true colors were exposed, Mairin decides to go back to the Lysandre Labs and rescue it with Professor Sycamore, Steven Stone, and Serena's help. But when the Megalith was activated by Lysandre, Chespie ended up getting absorbed by it and the Megalith was formed into a destructive version of a Zygarde. The Megalith Zygarde began traveling towards Anistar City, where it would get in contact with the giant sundial and once the two fuse together, it would create an explosion that would destroy the planet and wipe off nearly every living being. However, with some help from the Kalos gym leaders, Malva, Clemont, Meyer, and Diantha, Ash and Alain managed to reach the core of the Megalith Zygarde and rescue Chespie. After Squishy and Z-2 destroyed the Megalith Zygarde and killed Lysandre, Chespie awoke from its coma and Mairin was happily reunited with it.
|Vine Whip||Mega Evolution Special I|
|Pin Missile||Mega Evolution Special I|
|Toxic||Mega Evolution Special I|
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.