- This page is about the object. You might be looking for the episode of the same name.
Yui's Heart (ユイの心, Yui no Kokoro?) refers to an item containing the data for Yui's program, which is around three terabytes large. The item is officially known as MHCP001, which stands for Mental Health Counselling Program, by the system.
Yui's Heart is an object which has the appearance of a crystalline water droplet.
After Yui's existence was detected by the Cardinal System, Kirito utilized a system console in the Hidden Dungeon to search for Yui in the Sword Art Online server with the search command. He then extracted and compressed the program into his own NerveGear's storage, creating Yui's Heart as an object in the game. Asuna then used the item as a necklace.
When Kirito first logged into ALfheim Online, he noticed that his account shared data with his SAO avatar. He then quickly checked his item storage and, among all his corrupt items, he found Yui's heart. By selecting to materialise the object, the item was taken out of his inventory. Kirito then tapped on the item to activate it and it turned into Yui, the Navigation Pixie.