Yui is an artificial intelligence program designed to monitor and support the mental health of players in the virtual reality MMORPG, Sword Art Online. She is discovered by Kirito and Asuna in a forest during their in-game adventures. Initially appearing as a young girl with no memory of her origins, Yui quickly forms a strong bond with the couple, becoming a beloved daughter-like figure to them. Her character brings warmth and emotional depth to the series, highlighting themes of family, love, and technology.