Marisso is X's Chespin.
Marisso used to belong to Professor Sycamore, who gave Marisso, Fennekin, and Froakie to Trevor for X to choose between in the hope that it would rekindle X's love of battling and get him to leave his house.
Marisso continues to be with X even when X refuses to acknowledge him as part of his party. Trevor convinces them to try to see if Marisso can Mega Evolve, but their attempts fail.
It isn't until they meet Professor Sycamore in person and X and Marisso fight and lose to Sycamore and Charmander that X accepts Marisso as his own. (This is when X gives him the nickname "Marisso.")
- Pin Missile
Despite of X's indifference to him, Marisso fights as hard as he can for X. Marisso is also quite friendly with the other Pokémon, seen when he jumps into Kangaskhan's pouch alongside the baby Kangaskhan and when he greets Charmander.