Previously on Sword Art Online:
Kirito & Asuna, now married, take a break from the front lines and live together in a cabin on floor 25. While there, the encounter a mysterious young girl named Yui who has no memory of her past. Spoilers Ahead.
After traveling to the Town of Beginning to help Yui regain her memory, Kirito & Asuna soon find themselves defending a young woman named Sasha and some orphans from the Army who came to extort taxes from her. After helping them and returning to the Orphanage Sasha runs, Kirito & Asuna are requested by woman named Yulier, second in command to the leader of the Army, to help her find her leader, Thinker. When they reach the dungeon he’s locked in, Yui’s memories start to return.
This episode explains Yui’s origins, unfortunately the resolution of the storyline is very rushed. Yui is essentially an advanced AI program within SAO that was given emotions to be better able to communicate with the players and monitor their mental states during game play. When Kayaba trapped the players in the game 2 years ago, Cardinal (the game’s operating system) locked Yui from interacting with the players, forcing her to watch as their mental states deteriorated, some of them even committing suicide. Eventually she became intrigued in Asuna & Kirito for while everyone else was despairing about being trapped, those two were able to make due and be happy; which is why she went to the 22nd floor to meet them.
That is a lot for one to process, but it works well enough. The resolution of this revelation is another matter, after defeating a monster; Cardinal runs a system check on Yui and deems her a program for deletion, which obviously upsets Asuna & Kirito. Therein lies the problem with the episode, it upsets Asuna & Kirito, but not the audience. Her deletion is built to be a sad and dramatic moment, but 2 episodes isn’t enough time for the audience to establish an emotional connection to Yui, it just seems the scene is meant to be sad because she’s only a little girl (well AI program with motions but I digress).
The Army is fleshed out more here by Yulier. Stating that when Thinker started it their goal was to distribute info and food among the people, but as the group grew larger corruption started to spread. Eventually Kibaou (the loud mouth from Episode 2) ended up with a great deal of power and sent people to the front lines to fight, which led to the “Gleam Eyes incident”; fearing expulsion he set a trap for Thinker in the dungeon Kirito and Asuna explore. We also learn about the orphanage from Sasha, stating they were kids trapped in the game with the others. There are also a few good action scenes when Kirito takes out a huge group of enemies and he & Asuna team up to fight an overpowered monster (which Yui defeats).
“Yui’s Heart” while having a couple problems here and there, is still a fairy decent episode with just the right mix of humor, action, and drama to make it worthy of “Top 5 SAO episodes” list. Kirito & Asuna advance further as a couple, while also getting their individual development as well and the world is expanded more. The fight to survive as Asuna & Kirito return to the 22nd floor.