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 the GM Kayaba Akihiko to come close to the players at the beginning of 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. In ALO, she was able to return to her normal form because ALO was based on a copy of the SAO server. She supports Kirito there as a Navigation Pixie. Kirito and Asuna still treat her as their daughter.