When Kirito enters Alfheim Online and notices his stats are from SAO, he quickly scrolls through the list of bugged items looking for Yui's Heart. He found it but it was renamed to something else, and when he used it, Yui appeared.

Yui's Heart was a very unique item in SAO as it was created from what Kirito could save of Yui after she was deleted. Since REC Progress made Alfheim Online based off of SAO, it would explain how all of Kirito's SAO items were bugged but Yui's heart remained and was renamed.

Why was Yui's Heart, the most unique item in SAO, the only SAO item in Kirito's inventory which wasn't bugged?

  • IIRC, Kirito said something about storing Yui in his local memory. But that may have nothing to do with it.
    – Mysticial
    Mar 25 '14 at 22:36
  • 2
    @Mysticial he was referring to storing Yui in his NervGear's memory should SAO be cleared that way he and Asuna could be with Yui in the real world
    – Memor-X
    Mar 25 '14 at 22:38

I'm going to answer this from a programmer's perspective.

When writing a video game, everything is built in layers, starting with simple IO (mouse, keyboard, sound, video), then a physics engine is integrated, and then base game mechanics added.

Finally, the game itself is written on top of all this software.

All of Kirito's items would be part of the SAO game itself, and would be unrecognized item codes in another game.

However, Yui is a more integral part of the game engine, her program would exist in a earlier layer of the game. When the game was stripped down from SAO in order to make ALfheim, Yui's layer would have been left intact, allowing her to exist.

It is the same with Kirito's stats, they are more integral than the game items, and are programmed first.
(The fact that his stats transferred actually shows how sloppy & lazy of a rewrite ALfheim was)

Side note: In a game as advanced as SAO or ALfheim, it is possible that items are created as needed by the engine (the Cardinal), every game would have its own set of items, with unique IDs.


The ALO system categorized her ("MHCP001" in the menu) as a Navigation Pixie, a pseudo-character program for user support, which is similar to her function in SAO, as a "Mental Health - Counseling Program." Since ALO is based of SAO as a base, it's more then likely it recognizes Yui as a valid program type like a program recognizes if it can open valid file types.


This may be an hypothesis of mine, but the core of Alfheim Online, is the one of SAO. Yui was part of the core of SAO, or rather an auxiliary program of the core. That's why Yui was able to be restored in Alfheim. The other items probably were items from SAO only, which means are more than records than components of the core, and that's why they don't work.

Besides, plot-wise is much better that for a new adventure, you have to start from 0. It is not the level, or the items that makes the player, but the player's skill as a gamer. (Kirito's is very high, as it's seemed)


I don't think there's any way to confirm exactly why Yui's Heart is the only item to work in Alfhiem but what I believe is the reason is that Yui's Heart is an Item which holds information to give information to and initialize a program, now since Alfhiem is more or less created from SAO the program is likely to be carried over. Whereas the rest of the item's in Kirito's inv simply don't match the variables etc of the item's in Alfhiem.


I think it's because the ALO was a copy of SAO but with some different function. As you know all the basics of the game were same like movement id creation but with more races than SAO. The level upgrading and loot system were same too but that doesn't prove why Kirito's other skill were bugged so here's why SAO was a game that was not related to reality and but ALO it was mentioned that your game physical ability would depend on your real ability so kirito didn't got his other skills because he didn't have that ability in reality but in battle of ALO we've seen kirito use his duel weild skill. Kirito was able to use that skill because he practised it once with his sister [causin]. Now back to the topic why Yui wasn't bugged its because she was a part of SAO's basic programming code but she was just modified just like the monsters they didn't rewrite the programming code of monster's or Yui because that would require lot of time.

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