In They Have a Flare for a Li'l Kanga-Napping, Tierno presented Chespin and Froakie to X and revealed that he, Trevor, and Shauna agreed that Chespin and Froakie should go to X and Y, as they are the best battlers of the group. X refused both of them, as he does not want them to be constantly targeted and attacked by their enemies, Team Flare, due to them being on his team. Later, when the Team Flare scientists Aliana and Mable attacked the group in an attempt to steal X's Li'l Kanga, Chespin and Froakie jumped forward to protect the group and were attacked by Aliana's Diggersby's Mud Shot and Mable's Houndoom's Flamethrower. While Y struggled to figure out what to do, X teamed up with Chespin to stop the villains. With his speed and sharp quills, Chespin exploited Diggersby's strength to free X's friends from the room they were trapped in and then pinned Aliana and Mable to a tree, defeating them and giving the group the chance to escape. Despite battling together, X still refused to let Chespin join him team.
The group was later invited to the Super Training facility for the pre-opening and were given a free visit. X decided to have Chespin try it out, as he figured the Spiny Nut Pokémon would enjoy it. Chespin practiced on an Axew balloon, raising his attack. Team Flare later appeared and had their Noibat prevent the group from hearing and keeping them from giving their Pokémon commands. X uses nonverbal instructions and has Chespin pop a Wailmer balloon which then crashes into the Noibat and Team Flare, sending them flying away.
In What Does Charmander Do When It Dozes?, X and his friends met Professor Sycamore in person. While Trevor and the others went somewhere else, Professor Sycamore questioned why X did not let Chespin on his team. When X explained why, Professor Sycamore decided to have a battle and made X use Chespin while he used his Charmander. X agreed, but refused to actively take a part in the battle, forcing Chespin to fight by himself. With no Trainer and at a type-disadvantage, Chespin was dealt heavy blows, but refused to give up. Chespin's determination managed to impress X enough to actually take a part in the battle, and had him attack with Rollout. Despite the powerful attack, Chespin was defeated. After the battle was over, X decided to keep Chespin and named him Marisso, and also took the Charmander, which he nicknamed Salamè.
As a Quilladin
In Pangoro Poses a Problem, Marisso teamed up with Kanga and Salamè to battle the Lumiose Press editor-in-chief to protect Alexa. Marisso started the battle by slamming into Pangoro's side with his quills, but Pangoro's fur proved to be too tough to pierce. He then began to fight one of the chief's Spewpa, and due to it using Rage Powder, was unable to listen to X's commands. He eventually defeated the Spewpa, but was left exhausted, allowing Pangoro to grab him and Salamè with its paws. X then told Salamè to release his tail, which caused Pangoro's leaf to burn up. Without the leaf allowing it to sense what Kanga and Li'l Kanga were doing the two quickly defeated it.
In Hawlucha Attack, Marisso joins the rest of X's team, save Salamè, to fight against Essentia's four Hawlucha. Essentia soon flees on one of the Hawlucha and the rest lose their ability to stay in formation and get defeated.
In Chesnaught Protects, on their way to the Pokémon Village to confront Team Flare at their hiding place, X and his friends were attacked by Xerosic. After Xerosic's Malamar put everyone to sleep with Hypnosis, X was left with only Salamè to use in battle. Before he can do anything, X is attacked by Malamar, causing him to drop Salamè's Poké Ball. Just as Malamar attacks the Poké Ball, Marisso steps in to defend him, evolving into a Chesnaught in the process. Though injured from the event, Marisso's determination gave Salamè the confidence to start fighting. Marisso later tries to help suppress Mewtwo's rage at the Pokémon Village alongside Charizard, Rhyperior, and Salamè. Despite their numbers Mewtwo keeps the advantage, easily knocking them away with Psywave. In Mewtwo Angered, Marisso uses Pin Missile on Essentia, destroying her helmet and revealing her identity. Afterwards, Marisso acts as a support for X while his team-mates Mega Evolve all at once to fight Zygarde.
In Epilogue, Marisso battles against AZ's Golurk. Marisso protects the town from Golurk's barrage of attacks and convinces AZ to cease the assault after hitting him with a Pin Missile.
Personality and characteristics
Marisso is a very brave and daring Pokémon. He wastes no time jumping into difficult battles and refuses to give up, even when his Trainer shows no interest in the battle. He is also patient and calm, willing to wait until X was ready to accept him. Marisso believes in the potential of others, shown in his continued support of his Trainer and his childhood friend, Salamè.