Voiced by: Kanae Itō (Japanese), Stephanie Sheh (English)

Initially a side story character in SAO, Yui later becomes an accompanying/support character in ALO. At first thought to be a lost child in SAO, she is actually an artificial intelligence program. Her full designation is «Mental Health - Counselling Program», MHCP version 1, codename, «Yui». She was designed to monitor the emotions of players and appear at their sides to hear and help them out. However, since the moment everyone was imprisoned in Sword Art Online, she was forbidden to come in contact with the players and complete her prime directive. Unable to do anything but suffer as she monitored the emotions of the trapped players, she came upon Kirito and Asuna whose emotions stood out as a beacon of joy and peace in a sea of fear and despair. She sought them out, appearing to them as an amnesia child whom they adopted. In the end she was nearly deleted by Cardinal, the management system of SAO. But Kirito saved her by converting and storing her data into his own Nerve Gear. Yui later shows up in ALO as a Private Navigation Pixie and helps Kirito along with Leafa to find Asuna. She refers to Kirito as "Papa" and Asuna as "Mama" and gets very annoyed if any girl other than Leafa or Asuna gets close to Kirito. Kirito uses his skills and resources in the real world to construct a device for Yui to view the real world even while she is in virtual reality and gives her the ability to communicate with them.