Even if it was futile, everyone pitching in to try and save the world is both this and a Tear Jerker.
Mewtwo's confession in front of everyone about his love to Mew, even giving Legion permission to go as far as to kill him just as long as the others were free. Shortly then after, Legion wailed on him.
Which would lead to Mew breaking her wrist to stop Legion from killing Mewtwo.
How about Mewtwo in general? Legion had been trying his hardest to break him, and he still stayed strong because of his love for Mew. Even when he was beaten near-to-death, he went out of his way to slowly but surely climb the TreeŚrather hesitantly since his friends were at stakeŚto fetch Mew and give her the remnants of Clarity Lake that everyone else said he needed the most.