Attack on Titan has a lot of different mysteries, such as the origins of the Titans, the reasons why the Eldians were so hated, and even some regarding Eren Yeager's own family. In that regard, the conversation at the beach between Grisha Yeager and Eren Kruger is still discussed by the fandom to this very day.
A lot of people still asked themselves how Eren Kruger was aware of Armin and Mikasa's existence when he was talking to Grisha Yeager. Armin and Mikasa were not even born at the time this conversation took place, but the truth behind this question lies in the nature of the Attack Titan and how that connects to the two Eren characters in the series.
Disclaimer: This article contains massive spoilers for the Attack on Titan series.
Attack on Titan and the explanation of how Eren Kruger knew about Mikasa and Armin
During the events of chapter 89 of the Attack on Titan manga (or the end of the third season of the anime), Grisha Yeager, the father of the protagonist, Eren, is seen talking to the Eldian spy, Eren Kruger. This is long before the events of the main storyline, even before the main trio, Eren, Mikasa, and Armin, were born.
However, as Eren Kruger is telling Grisha about what the latter has to do to save the Eldians, the former also mentions the names of Mikasa and Armin. Of course, a lot of fans asked themselves how he could have known about these two characters when they weren't even born yet.
The answer is this: Eren Kruger was the wielder of the Attack Titan and this allows him to see the memories of past and future users. This, of course, includes Eren Yeager, who was the future Attack Titan user. And it is also worth pointing out that Eren himself traveled to the past and could have influenced Kruger's mind and memories to give the former's future self greater motivation to move forward with his plans.
The importance of Mikasa and Armin
Armin and Mikasa's importance in the Attack on Titan series cannot be understated, especially considering the element of Eren Kruger's words to Grisha. They are, in many ways, what anchored Eren Yeager throughout most of the series, which was also his motivation to find solutions to the problems the Eldians were facing.
While is true that Eren eventually descended into insanity and his decisions were wrong, there is no denying that Armin and Mikasa were perhaps the last thing he was latching onto when it came to his own humanity. It was after he started to drift away from them that he became a lot more unhinged and willing to take out his resentment towards the world.
These two characters were also pivotal in the aftermath of the Attack on Titan ending because they were involved in rebuilding Paradis. They had gone through hell and back, which is a big example of how much they had to overcome in order to have a semblance of peace in their world.
Final thoughts
Attack on Titan has a lot of interesting mysteries, and that conversation between Grisha Yeager and Eren Kruger lingered in the minds of fans for quite some time. However, it was the influence of the Attack Titan that allowed Eren to know Mikasa and Armin's names.