Mewtwo's Trainer, Stephanie Heartfilia
36 parts Complete Au- Mewtwo is staying with Ash along with Mew and Newtwo, Mewtwo also had a trainer before meeting Ash.
What was supposed to be a peaceful night at the Ketchum Household turned into a life or death situation when Pokémon show up at the door, one's that Mewtwo recognizes. With the information of someone being hurt and abused, Ash, Goh, Mewtwo and Ash's friends help Mewtwo's set free his trainer by saving her.
(Some Credit goes to OVAS Pokémon Comic, love your work!)