Yui

Yui (ユイ) is an Artificial Intelligence (AI), found by Kirito and Asuna around the forests of the 22nd Floor of the Floating Castle Aincrad.

Her official name is «Yui-MHCP001» (Mental Health Counseling Program 001).

Appearance
She has long dark hair and bright black eyes. In her most common form, she wears a white one-piece dress. In her Navigation Pixie form she wears a long light-pink dress with detached sleeves and pink anklets. In both forms she goes barefoot.

Personality
As Yui was originally a girl without her memories, she felt insecure about not knowing who or where her "parents" were, so she assumed that Kirito was her father and Asuna was her mother. After the couple decides to play along as the girl's parents due to the girl's psychological condition, she begins to be very cheerful and shows great affection for her so-called "parents". According to Asuna, her behavior was like that of a baby, despite her young appearance. However she does not show any fear, regardless of the kind of danger she might be in and even shows signs of rebellion when Kirito tells her that she has to stay with Sasha when Kirito and Asuna wanted to go to a dungeon to rescue Thinker.

After regaining her memories, she tells her in-game parents that she had become melancholic when the death game began, as she wasn't allowed to comfort players and was forced to continue monitoring their psychological parameters, but, after nearly 2 years of only seeing insanity, misery and anger, she finally noticed a couple that showed happiness and joy and wanted to be close to the two to experience their emotions, even though she was not allowed to interact with players. After being comforted by Kirito and Asuna, she regains her joy, even though she was about to be deleted by the Cardinal System, and showed no sign of repentance, but tears of happiness, which she had due to being with her in-game parents, instead.

Upon being reactivated in ALfheim Online she still displays a deep attachment to Kirito and Asuna, continuing to act as a surrogate daughter. She also scolds Kirito when he receives affection from other women aside from Asuna, stating her "papa" is "cheating," but she does, however, eventually accept Leafa into Kirito's life.

Morning Dew Girl
Hearing a rumor about a "ghost" that has appeared on the forests around the 22nd floor, Kirito and Asuna decide to check it out themselves. There they meet a young girl who suddenly falls unconscious. Initially, Asuna thinks that the girl was the ghost, Kirito rushes to Yui without hesitation. They immediately take her back with them and are faced with several concerns about her. They deduce that she's not an NPC, as she doesn't have a quest flag, and that picking her up did not result in a NPC harrasment warning. She also lacked a player cursor, which every player has and is far too young to be a normal player as setting up the NerveGear signup forbid children below thirteen years old. Later, during a quest to find the «Army's» leader, Thinker in the dungeon of the «Black Iron Palace», Yui touches a stone cube at the center of the safe zone at the deepest part of the dungeon and immediately regains all her memories. She then proceeds to defeat The Fatal Scythe using the GM console's «Object Eraser» function.

Afterwards, Yui, now knowing her purpose as an AI who will be erased now that the system recognizes her as an "alien" due to disobeying the system's orders, expresses her gratitude to Kirito and Asuna for taking care of her. Just as she disappears, Kirito uses the still active admin access and his computer skills to retrieve her and store her core programming as an in-game item, shaped like tear crystal. The item was named «Yui's Heart» and Yui was saved as environmental data on Kirito's NerveGear.

Fairy Dance Arc
After SAO, Kirito unknowingly transfers his SAO data to ALO. Most of his data is deemed unusable and corrupt, but he manages to retrieve Yui from his item window (she was listed as an item named «MHCP001» (Mental Health - Counseling Program) in the item window).

Here, she is classified as a navigation pixie by the system, and she leads Kirito to the proper locations throughout the land. Yui also finds information, mainly maps stored within the game's database, along with detecting incoming players and giving some advice when needed to help Kirito find Asuna. She is still able to go back to her regular body, but only does so when alone with Kirito and Asuna.

With Yui's help Kirito was eventually able to unite and rescue Asuna from Sugou Nobuyuki (Oberon). When they met Asuna again, Yui returned to her regular body, and they shared a "family hug". Unfortunately, before they could escape, Sugou intervened and attempted to delete Yui, but she managed to escape to Kirito and Asuna's relief. Yui attempted to reunite with Asuna and Kirito afterwards, but she was only able to, after Kirito successfully destroyed Sugou's avatar and helped Asuna log out of the game.

While Yui was sad that Asuna had already left, she understood why Asuna had to return to the real world. Kirito promised Yui that both he and Asuna would visit her soon. Before Kirito left to meet Asuna in the real world, Yui informed Kirito that a rather large file («The Seed») had been transferred to his NerveGear's storage. Yui also informed Kirito that regardless of what happens to ALO, she will be fine because she always exists in Kirito's NerveGear.

Sometime later, Yui joined Kirito, Asuna, and their friends on a new adventure in ALO.

Phantom Bullet Arc
Prior to Kirito entering GGO, she is mentioned in a conversation between Asuna and Kirito about the possibility of her having a "younger sister" in the future and how great it would be if Yui could cross over into the real world.

Later, during Kirito's final battle with Death Gun, Yui connects the Bullet of Bullets tournament broadcast to the heart beat monitor via Asuna's cell phone, letting Asuna and Nurse Aki see what is currently happening. Seeing that Kirito is losing the fight with his opponent, Yui tells Asuna to hold his hand, which helps Kirito remember of the gun that was still in the holster at his side.

Alicization Running
After Kirito is critically injured by a former Laughing Coffin member, Johnny Black, and subsequently kidnapped by Kikuoka Seijirou, Asuna and her friends began to think there was no hope in locating Kirito. However, Yui convinced her mother not to give up, just like when Asuna was trapped by Sugou, Kirito refused to give up on finding Asuna. It was enough to restore Asuna's resolve and with Yui's help, along with Koujiro Rinko (the deceased Kayaba Akihiko's lover), Asuna was eventually successful in finding Kirito.

Hardware
Yui is the first version of the «Mental Health - Counselling Program» of «Sword Art Online». She is a «Top-down AI» made to monitor the mental health of the players of SAO. She was made to be able to mimic emotions and be able to interact with the players of SAO. During the events of SAO she becomes self-aware and a fully realized artificial intelligence built from a program atop an existing computer architecture, which is why she usually appears in programs based on SAO. Top-down AIs can be considered as a cluster of binary.

She later gains access to the real world through a VR probe (Audiovisual Bidirectional Communication Probe) designed and created by Kirito.

Her core program runs and is stored on Kirito's NerveGear local memory.

Trivia

 * Yui's actual main thought routine is approximately 3 terabytes. The capacity of her external storage is unknown.
 * Yui is also capable of accessing search engines and computer networks. Her thought processes are extremely effective, having an extraction rate of 95% of useful data.
 * So far, the only girls in the entire series that Yui likes that are around Kirito (beside Asuna), are Suguha and Shino.
 * Yui thinks that Kirito's most annoying habit is him napping anywhere and everywhere, as she gets sleepy from them too.
 * Yui enjoys communicating with Pina, as the familiar's intelligence is of a complex structure.
 * Although Yui would like a younger brother or sister, so that she would have someone to talk to, she also wishes to monopolise her Papa and Mama.
 * Yui is able to reproduce her original body in-game, but only uses it when sleeping together with Kirito and Asuna, as her fairy body is more comfortable in many ways.
 * Yui is able to hear other player's alarms.