"Her back story is she was pretty much raised as a human by a human family, when she was a young mew she was dieing and her last option to call for help was to change into a human. Since no one knew who she was after she was healed she was adopted by her parents. So when she reached her tenth birthday she obviously would get her first pokemon, as the pokeball she has but her 'parents' hated pokemon but wanted her to experience new things so they let her go on her journey. She is afraid of what her parents would do if they knew she was a mew. She also feels strange being a pokemon pokemon trainer. xD but she has forgotten most of her mew ways thus acting more human like then pokemon like."
http://poolpelt-nightflower.deviantart.com/art/Purple-mew-394679893 