For those who do not know, in the Movie and in the Audio Dramas, Mewtwo had a really really close friend who taught him and cared for him as a mother or an older sister would have.
The girl known as Aitwo, or Ambertwo in the dubbed version of the "Birth of Mewtwo" special which was cut from the original releases of the dub and not released in its intirety until later times, was the cloned daughter of Dr. Fuji, who had been killed in a car accident, bringing Fuji to an obsession to see his daughter again, even to go as far as to join the notorious Team Rocket so he would have the funding to feed his experiments. She and Mewtwo became close friends during their time in the tubes in the laboratory on New Island, teaching him to count, teaching him about life, that life is wonderful. However human life force is not as strong as that of Pokemon and she was doomed to fade, and as her body began to disintegrate, Mewtwo experienced an emotion that many have not seen, sadness... And with the loss of her, he was upset, and his immense power forced Fuji to tranquilize and erase the memory of the young Mewtwo, leaving the faint memory of her last words, Life... is Wonderful...

I do not own any of the characters, or trademarks, Pokemon, Mewtwo Aitwo, Ambertwo, Team Rocket, Dr. Fuji; they are all properties of Nintendo, and the animation companies licensed to make the movies and series.