Turns up I managed to give it a role :3

Mewtwo
Gender: ?????
Current Residence: Unknown Dungeon, Kalos
Nature/Characteristic: Timid / Often lost in thought
Movepool: Psystrike - Brick Break - Recover - Miracle Eye

An artificial, man-made, false deity, Mewtwo is a powerful being which power out-muscles all known pokemon. Even almost all Divine Legendaries are no contest against it when uses it full potential.
Known by very few humans and by none of the Legendaries, it was genetically created several years ago through an eyelash of an undetermined now-deceased Mew somewhere in Kanto.
After destroying and fleeing from its birthplace, Mewtwo took refugee in Cerulean Cave where hid for a long time until the ex-Kanto champion named Red, back then newly promoted as such, found and defeated it. Avoiding potential capture, it once again departed and now it resides deep inside the Unknown Cave in Kalos, where found it respective mega stones.
Though it's still very sore of humans after what happened, Mewtwo chose to live a solitary and pacific life in the cave, away from most eyes and avoiding any sort of conflict so its power wouldn't ever go out of hand, specially as now holds both mega stones.

As usually pokemon can only archive Mega Evolution though a trusting relationship with their trainer, such is not Mewtwo's case. It can mega evolve without such requirements, similarly as Rayquaza. Although how that's possible it's unknown. Plus it didn't mega evolve even once so far.

That's likely because if it does, it existence won't be no longer a secret. It split mega evolution has the same source of power as the two mega charizard species: Xerneas and Yveltal.
What could happen if Mewtwo decides to take advantage of such power.