Yui is a supporting character in the Sword Art Online series. Originally designed as a top-down AI program called Mental Health Counseling Program 001 (MHCP001) for Sword Art Online, she was meant to monitor and assist players with mental health issues. When SAO became a death game, Yui was prevented from interacting with players, causing her to malfunction due to the conflict between her purpose and restrictions.

Found by Kirito and Asuna on the 22nd Floor of Aincrad, Yui was initially mistaken for a lost child player. After regaining her memories and saving Kirito and Asuna from a powerful monster, Yui was nearly deleted by the Cardinal System. However, Kirito managed to save her core program as an in-game item. Later, in ALfheim Online, Yui was reactivated as a Navigation Pixie, continuing to support Kirito and Asuna in their adventures.

Personality and Appearance

In her human form, Yui appears as a young girl with long black hair and dark eyes. She typically wears a simple white one-piece dress with a ribbon around her waist. In ALfheim Online, her Navigation Pixie form is about 10 centimeters tall, with a pink dress and detached sleeves.

Yui's personality is cheerful, curious, and deeply affectionate towards Kirito and Asuna, whom she calls "Papa" and "Mama." Despite being an AI, she displays a range of emotions and a strong desire to understand human feelings. Yui is highly intelligent and can access vast amounts of data, which she uses to assist her adoptive parents and their friends.

Powers and Abilities

As an AI program, Yui possesses several unique abilities:

• Vast Knowledge: Access to extensive game data and real-world information.
• System Access: Can interact with and manipulate game systems (when not restricted).
• Emotional Analysis: Ability to read and understand human emotions.
• Immortal Object Status: In her original form, immune to in-game damage.
• Form Shifting: Can switch between human and pixie forms in ALO.
• Cross-Game Compatibility: Able to exist in multiple VR worlds.
• Real-World Interface: Can interact with real-world devices through special hardware.

Trivia

• Yui's name in katakana (ユイ) could be interpreted as "User Interface."
• She was originally designed to appear as a hologram to counsel players in SAO.
• Yui can reproduce conversations she's heard, helping players review NPC dialogues.
• In ALO, she dislikes sitting on Kirito's head when his hair is spiky.
• Yui has expressed interest in having a younger sibling.
• She enjoys breaking through firewalls as a hobby.
• Yui can hear other players' alarms in the game.
• Her core program is stored as an item called "Yui's Heart" in Kirito's inventory.