When the portal that could send the Guardian back to the past opened in the future Heavenhold, this Guardian Knight, among all the Knights in the infinite number of parallel universes, decided to "stay in the future world" and spent 10 years since.
As the Knight gazes upon the portal, the Knight did know this future can be prevented. And that the Little Princess from 10 years back would not be abandoned. However, the thought that the Future Princess, who just finally coped with her past, might cease to exist, and the thought that the 10 years she and her friends had to endure without the knight would be for nothing. So the Knight made a decision. A decision to stay in the future and continue the battle.
And 10 years have passed since. Battles became way harder than the showdown with the Dark Magician waged on. Turning the table of the war seemed impossible. Most of the Earth was still under-occupied by Invaders, and not many Resistance were alive. However, the Resistance did not disperse or give up. With the trust that the Knight would not abandon them again, the Resistance rallied behind the Knight and Princess and made small but precious progress in taking back some of the lands. This was the reason why the Knight could comfort the Princess with confidence, who was blaming herself for not letting the Knight go back to the past. With people fighting alongside, the Knight doesn't regret the decision to stay in the future. There are still many hardships to overcome, but the Knight, the Princess, and the rest of the Resistance rely on each other to take another step forward.