Yui

A young and cheerful girl with long black hair who looks no older than 10 years old. Kirito and Asuna found her in a forest in the 22nd floor of Aincrad. After taking care of her, she appears to have lost her memory. She calls Kirito and Asuna "Papa" and "Mama" due to having trouble pronouncing their names which is a side effect of the memory loss. They decided that they would go and search for her friends in the Starting City. Her true identity is actually a players' self-regulation program for mental maintenance, Mental Health Counseling Program or MHCP. After being forbidden by Cardinal from coming close to players during the death game, she was flooded with errors and broke down. In the process she became an AI that possesses actual intelligence. When the Cardinal system governing SAO decided to erase her, Kirito turned her into a tear shaped crystal pendant. He actually converted her to a part of environmental data for the client program and was saved to his NERvGear's local memory.