So is a Hasty Mewtwo good in Let's Go? many players prefer Timid to boost SPD or Modest for SP ATK. But I think Hasty is not bad either you get boost for SPD and lose little DEF while keeping ATK and SP ATK the same, this way you can set move sets for both of them any time.
It's good on cart to be able to choose mega X or Y on the spot; Mewtwo usually carries Brick Break for fighting STAB because of this. Dedicated calm mind mega Y is also great, though.
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