Mewtwo being from Pokemon Quest means he only has 2 certain moves (2 moves from his learnable moves in Quest: Hyper Beam, Giga Impact, Psychic, Recover, Thunderbolt, Bulk Up, Blizzard, Shadow Ball, Psytrike, Psycho Cut) that he can use in battle and has to wait a short amount of time before using another move, similar to Sans' stamina. Mewtwo can not learn Swift nor can they Mega Evolve in Quest. This makes the fight much more balanced than a normal Sans vs Mewtwo matchup considering Mewtwo can learn Swift and can Mega Evolve without a Mega Stone.