Yui (ユイ) is a young and cheerful girl with long black hair who looks 10 years old. Kirito and Asuna found her in the 22nd floor forest in Aincrad in SAO game. After taking care of her, Yui appears to have lost her memory.
She thinks that Kirito and Asuna as her parents by calling them as Papa and Mama because of having problem pronouncing their avatar names which is a side effect of her memory loss. After Kirito and Asuna found out her real identity, they decided to take care of her just like their real daughter.