Yui

Yui is actually the Mental Health Counseling Program, an MHCP created to support players psychologically. After being forbidden from approaching players, accumulating errors eventually turns her into an AI with genuine intelligence. When Cardinal moves to erase her, Kirito converts her into environmental data and saves her in his NerveGear's local memory. In ALfheim Online, she regains her normal form as a Navigation Pixie, while Kirito and Asuna continue treating her as their daughter.