Kirito and Asuna may be Sword Art Online’s central duo, but they're separated for the vast majority of the Alicization arc. With Kirito in the new VR realm Underworld and Asuna trying to track him down in the real world, the couple had their own separate adventures that provided plenty of plot twists. But due to a climactic twist, the partners got plenty of time to catch up -- 200 years, in fact.
Near the end of Sword Art Online: War of Underworld, Kirito and Asuna chose to stay in Underworld to help ensure Alice's escape to the real world. That decision subjected them to 200 years spent in the VR realm as they were caught in Underworld’s reboot, during which the Fluctlight's technology greatly accelerated time. When Kirito and Asuna finally return, they decide to erase their memories of that time. Let’s explore why they make this choice and what happened during those two centuries.
Why Asuna Decides to Join Kirito For 200 Years
To explain the time skip more fully, Fluctlight acceleration makes time pass over 1000x faster in Underworld than the real world. Trapped in Underworld for multiple years during the Alicization arc, Kirito was isolated from everyone he knew. That’s why it was especially heartbreaking to see him accept a sentence of another 200 years alone in Underworld during the finale.
In an act of true love and devotion, Asuna suddenly appears before Kirito, willing to stay behind and experience the centuries together. She assures him that side by side, they could even take on a thousand years together. As they reunite, they also promise not to forget Alice or their many friends on the outside, but it’s unknown how thoroughly they’re able to retain such memories. Kirito does address Alice when he wakes, informing her that her sister Selka has entered Deep Freeze to wait for Alice.
What Happened During Asuna and Kirito' Underworld Lifetime?
The 200 years Kirito and Asuna spent in Underworld is glossed over, with Sword Art Online showing them waking up in the real world shortly after. A mere slideshow depicting post-war negotiations and parades gives fans a hint about what happened, but those events are otherwise largely veiled, hopefully to be explored in a later chapter. However, there is another aspect of their opaque experience conveyed through dialogue: Kirito and Asuna became royalty.
This is first revealed when Kirito refers to Asuna as “her highness” when they wake in the real world. When Rath programmer Takeru Higa makes a copy of the 200-year-old Kirito Fluctlight, he questions him further about their experience. Kirito confirms Asuna was Queen and that they discussed the necessity of a memory-wipe and the possibility of their Underworld experiences being copied. Above all, 200-year-old Kirito vows to fight for Underworld's rights so the real world will keep it alive.
Kirito and Asuna were known as Star King and Star Queen of Underworld, ruling the land for their 200 years spent there. The details of their reign are unclear, but they did proclaim they intended to help out the “new world” that Underworld became after the war. Their devotion to Underworld and knowledge from the real world likely made them passionate leaders, but could their political interest truly withstand 200 years?
Time Takes Its Toll on Kirito and Asuna
Rath easily erased 200 years' worth of Underworld memories per their request, but why was it necessary? While the mystery is meant to leave the audience wondering, there’s plenty of sound conjecture to explain why Kirito and Asuna wanted to leave their Underworld memories behind.
Upon reawakening in the real world, Kirito says in a jaded voice that he and “her highness” are “done there” and that they want their memories wiped. This Kirito sounds drastically different from the one from before, with the weight of two lifetimes aging his tone and manner of speech. From this alone, it’s clear that living through 200 years significantly drained them psychologically and emotionally.
The lifestyle of a ruler isn’t exactly relaxing either -- it’s likely Kirito and Asuna went through many hard times as King and Queen with new challenges arising in Underworld over the course of two centuries. It’s entirely understandable that one might want to forget such testing times.
Moreover, going back to their normal life with 200 years of memories riding on the back of their minds seems nearly impossible. If they kept their Underworld memories, how could Kirito and Asuna possibly remember all their adventures with their many friends throughout Sword Art Online? While the two seem rather choked up about sacrificing the most uplifting moments, they find comfort knowing that “deep down inside the memories are always there.”
Human Minds Have Limited Memory Storage
Even if Asuna and Kirito had desired to hold on to their memories from their stay in Underworld, it’s possible they simply couldn’t. A popular rationale for their memory erasure is that after two centuries, they'd reached memory storage capacity. In Sword Art Online, there’s a limit to the data storage and processing of a human mind.
This concept is first shown in Sword Art Online: Alicization when Quinella, also known as Administrator, approached 150 years old. From losing consciousness to forgetting Sacred Art commands, the years of built-up memories actually started to inhibit her ability to process her existence. Quinella found a way around it by removing old memories and backing them up via Cardinal, but her human memory limit remained. This capacity may actually reflect that of someone from the real world as well.
After 200 years, Kirito and Asuna have clearly filled their data storage, meaning they’d have to start choosing which memories to erase or face certain functional drawbacks. Thus, their decision to scrap those memories and start fresh seems like a practical and necessary form of personal maintenance. Luckily, Takeru Higa's copy of Kirito’s Fluctlight seems to have retained all these memories and stored them separately.