Yui is an artificial intelligence program created as the Mental Health Counseling Program for Sword Art Online, designed to monitor and support players' mental health within Aincrad. Due to Cardinal System modifications banning direct interference, she was isolated and took human form as a child-like fairy. Encountering Kirito and Asuna, she bonds with them as their 'daughter,' gaining emotions and love. After SAO's end, her core survives in a pot on Yggdrasil, enabling reunion in ALfheim Online. Physically resembling a 10-year-old girl with brown hair and eyes, wearing a white dress, Yui assists by providing game information and hacking support while cherishing her family.