Sword Art Online revolves around the concept of transferring one’s consciousness to virtual reality worlds to embark on thrilling adventures. The main characters, Kirito and Asuna, who are romantically attached, witness a huge change in this during the Alicization Arc.
The plot changes, focusing on Kirito's exploration of the new VR world he encounters and Asuna's quest to find her beloved. Luckily, due to a couple of fortunate events, the duo are reunited and spend a long time together, reaching up to 200 years.
What Happens in Sword Art Online: War of Underworld?
Towards the later part of Sword Art Online: War of Underworld, a situation arises, forcing Asuna and Kirito to stay in the Underworld to aid Alice’s escape from the VR world to the real world. As a result of their choices, they spent 200 years in Underworld, a situation caused by VR’s reboot. The 200 years they spent in Underworld were attributed to Fluctlight's technology effect; time acceleration. After Kirito and Asuna leave the Underworld after 200 years, they decide to remove their memories of the VR world.
Many SAO fans were puzzled at their decision, but Asuna and Kirito made their decision with good reason. Regarding the Fluctlight's acceleration, time was manipulated unusually. In short, Fluctlight's technology caused time to speed up by 1000 times compared to the real world’s time flow. Asuna also decides to stay with him, since earlier on in the Alicization Arc, Kirito was trapped alone, away from his friends, lover and family. So, Asuna convinces Kirito that they can spend even a thousand years in the Underworld together. Kirito and Asuna vow to remember Alice and her friends, waiting for them in the real world. Surprisingly, after leaving the Underworld, Kirito recognizes Alice, despite having 200 years’ worth of memories.
Later on in Sword Art Online, Asuna and Kirito move back to the real world after their lengthy unintended exile. However, at this point, the anime only shares brief information concerning what happened within the bi-centennial period. A scene showing post-war deliberations and events, such as Gabriel’s defeat, offers a slight insight into what truly happened. Apart from this, nothing major is revealed. Despite the absence of visuals regarding what happened, conversations in the real world revealed a big deal. The first is that Kirito and Asuna were royalty in Underworld. This can be deduced because soon after waking up in the real world, Kirito unconsciously calls Asuna “her highness.”
The anime reinforced this assumption when Takeru Higa, the Rath programmer, produced a copy of Underworld's 200-year-old Kirito Fluctlight. Takeru asks Kirito to narrate his experience, and he replies by saying Asuna was Queen, confirming suspicions. Afterward, they discussed the importance of erasing Kirito and Asuna. Rath also shares the possibility of copying their experiences in the Underworld. Honorable Kirito then declares his intention to fight for the Underworld, which is quite understandable after living there for two centuries.
Why Do Kirito and Asuna Want Their Memory Erased?
In the Underworld, Kirito and Asuna were the king and Queen of the VR world, leading the society throughout the period they lived there. Although specifics of their reign were only revealed partially, SAO fans can infer that their reign was borne out of an act to help the Underworld during the post-war period.
Later on, per their request, Rath erases the memories of Asuna and Kirito. There have been several theories regarding Kirito and Asuna's reason for requesting a memory wipe. After Kirito and Asuna's awakening, they declare that they want their memories wiped, but through keen observation, fans can observe a drastic difference between Kirito from the Underworld and his original self. Undoubtedly, Kirito and Asuna faced a lot of changes in the Underworld which taxed them emotionally and psychologically. After all, having 200 years’ worth of experience being royalty to an unusual society is no small feat. So it’s possible that they wanted to be freed from the emotional burden they carried.
Also, another possible theory is that the human brain cannot hold more than a specific number of years' worth of memories. This was seen in Sword Art Online: Alicization, where Quinella, alternatively known as the Administrator, had 150 years’ worth of memories that soon affected her cognitive abilities. Even though she invented a temporary solution: removing parts of old memories to gain new ones, it was a far cry from a suitable one since her primary human memory was still limited. Consequently, if Asuna and Kirito had retained their memories, they might have been in a similar situation, involving a temporary removal and replacement of their memories. Following this thought process, permanent and complete memory erasure of their Underworld experience was a practical decision. Luckily, Rath’s copy of Kirito saves stores his memory for future references. Sword Art Online fans would be eager to discover the adventures and situations Kirito and Asuna experienced in Underworld if the anime ever veers toward that path.