Yui

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.