Chespie first appeared in Mega Evolution Special I where it along with Mairin watched the battle between Alain and Astrid. After the battle, Chespie and Mairin went up to Alain when Mairin wanted to discuss to Alain about the different forms of Charizard and Absol. Chespie and Mairin followed Alain around while he battled various Mega Evolved Pokémon. Later, when Mairin encountered a Yellow FlowerFlabébé, she had her Chespie battle her in order for Mairin to catch her. Chespie had trouble in the beginning as Flabébé was very fast. After Alain gave Mairin advice, Mairin had her Chespie use Toxic which weakened Flabébé enough and Mairin was able to catch her.
In Mega Evolution Special III, Chespie accompanied Mairin while she sneaked aboard a helicopter as Alain forbid her from coming. It protected Mairin from ice shards caused by the rampage of PrimalGroudon and Primal Kyogre who was trying to get a knocked out Alain to safety.
In Mega Evolution Special IV, Chespie accompanied Mairin to Lysandre Labs. After Alain refused to allow Mairin to travel with him, she asked Chespie to give her a moment alone. This resulted in Chespie following two scientists to a room where it found a Zygarde Core (Squishy) and some cells being subjected to some sort of evidently painful experiment. Chespie rescued Squishy after the equipment overloaded, but the energy Zygarde had been subjected to caused Chespie to fall into a coma, devastating Mairin.
In the main series
Chespie made its first appearance outside the Mega Evolution Specials alongside Mairin in A Meeting of Two Journeys!, at Lysandre Labs. Alain called Lysandre for an update on the situation, only to be informed that nothing had changed at the time.
It appeared again in The Synchronicity Test!, where Mairin sent Alain a video of her and Chespie. Though Chespie had shown no signs of improvement, she assured Alain that the two of them were fine, and told him to do his best. Lysandre claimed that Alain could revive Chespie by collecting Mega Evolution energy. In truth, Lysandre had no intention of helping the Spiny Nut Pokémon; the energy was actually to power a machine to influence the mind of Zygarde. After Lysandre's true colors were exposed, Mairin set out to rescue Chespie from Lysandre Labs, with the help of Serena and Professor Sycamore. Chespie was eventually rescued in The Right Hero for the Right Job!. However, it ended up absorbed by Lysandre's fallback plan, a Giant Rock with the appearance of Zygarde 50% Forme, in Rocking Kalos Defenses!, although it was eventually rescued by Ash and Alain in Forming a More Perfect Union!, temporarily halting the Giant Rock. The energy was absorbed by the Giant Rock, and as a result, Chespie finally recovered. Squishy thanked it before leaving with Z2.
Chespie tends to save its Trainer from trouble and helps her when she falls over. Chespie is also shown to be friendly with other Pokémon such as when it fist bumped Steven's Metagross after its battle. It shares some characteristics with Clemont's Chespin, in the sense that it can act goofy and be clumsy, similar to its Trainer. However, it has not been seen as gluttonous or mischievous. Similarly to Mairin, Chespie is a curious Pokémon with a strong sense of justice. It has been shown on several occasions to care deeply for its Trainer, using Vine Whip to prevent her from falling down and protecting her from harm.