Mewthree (Mega Mewtwo)

MewThree (or mega mewtwo in the games) was a Pokemon introduced in Pokemon X and Y

Partner
Mewthree was not raised alone. He was raised with his brother (most likely adopteted) Snorlax. Both of their trainers would even send them to infiltrate the mushroom kingdom.

The mushroom kingdom
During a presumable war with the mushroom kingdom two trainers sent their Pokemon Mewthree and Snorlax to infiltrate the mushroom kingdom and find Mario. Little did they know it may be the last time they will ever see them again.They took a couples minutes to get to the door. They would both die here. But they would respawn in the tick tock clock room, or as Bethany puts it "the star room". Mewthree arives only to see two evil Walilords that had betrayed the Pokemon kind joining Mario, who was near the entrance to the tick tock clock level. Mewthree is killed by the two waillords, but is revived by them to fight snorlax as a zombie. He failed and was crushed to death by snorlax.