If held by an Mawile, allows it to Mega Evolve into Mega Mawile if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Mawile a Mawilite or remove a Mawilite from Mawile.
Outside of battle
If held by an Mawile, allows it to Mega Evolve into Mega Mawile when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Mawile will remain a Mega Mawile until the Talent Round ends.