Summary: Eren Yeager and the Demons of Paradis win their war against the Marleyans, but at a great cost.
The Year 859. Shiganshina.
Morning, Eren! How do you feel?
It won't be a good morning, because every day you'll wake up with a killer headache. Every day, you'll wake up from what feels like a long, tiresome dream, and you'll never remember just what happened in that dream. You'll be frustrated if you can manage to think through the pain, but that's not always a guarantee.
Well, Eren – you wrote this letter in 855. You'll be dead in 859; here's why:
In 846, Reiner Braun, Bertholdt Hoover, and Annie Leonhart came from across the ocean and broke down Wall Maria, the wall around your old hometown, Shiganshina, where you are now. They were Titan Shifters; in other words, they were humans that could turn into Titans.
But guess what? So were you. So are you. You are the Attack Titan, the Shingeki no Kyojin; you are the Founding Titan, that which lords above them all.
But the thing is, Eren: Titan Shifters only live 13 years after gaining their powers. You gained yours in 846. They call it the Curse of Ymir, but they seem uncomfortable talking about it. Maybe you've never asked before, but anyway, that's why you'll die in 859. So cherish today.
It's a long story, Eren, and they haven't told you much more than this. You saved the world in the end; you defeated the enemies, the Marleyans, although you don't really know who they are or why they were your enemies in the first place. It came at a cost, though. Every day, you wake up with nothing. No memories, nothing in your brain but the pain of the morning and a dream that scurries away from your outstretched fingertips. Your friends will only tell you just about as much as is mentioned above. They will, however, tell you about themselves, because they've decided that in the 20 odd hours they have with you, that's the most important thing.
Sasha Braus is a feisty one. She has a fire in her eyes and a big appetite; she apparently catches all your food, so be grateful to this one. Don't bite the hand that feeds you, unless you consider that your own. Don't bite your own unless you want to shift. Your call.
He's not there, but they talk about a little pipsqueak named Connie Springer. He died in the fight against the Marleyans, but they tell you he had attitude that overflowed from the brim of his 158 centimeter frame. They've shown you pictures – he really does look like the size of your little titan finger.
Jean Kirstein is there, but he's all sullen and quiet all the time. They say he's changed a lot since you last remembered him. It caught up to him, a little bit; trained and conditioned to kill Titans, he had a lot of trouble with the aftermath of the human slaughter in the Liberio district (which is, by the way, the site of your final battle with the Marleyans).
Floch Johansen doesn't really seem to like you. He's like Jean, but at least Jean is a little nicer to you when you first wake up. Floch apparently fought by your side in Liberio, but he doesn't seem particularly close to any of your friends. It's a little bit of a mystery as to why he's here. According to Armin – we'll get to him in a sec! – the six of you are the only people still living within the walls. Today, you think it's for your sake. It must be lonely.
Levi Ackerman. They call him humanity's finest soldier. He was your first meaningful commanding officer and, so they say, your biggest fan, though the way they laugh about it makes it seem like they're poking a bit of fun at you, their old friend. He apparently died saving Floch's life in Liberio. Ah… maybe that's why Floch is here.
Hange Zoe. She's also not here. She died in 854, but after the Battle of Liberio. She had some sort of disease, and nobody could save her in time. She's famous for her intellect; she developed the technology that apparently won you a ticket off the island of Paradis (where Shiganshina and the Walls are located). They say she was a compassionate leader as well, wise beyond her years.
They mention some other names too, but mostly in passing. Marco Bott is one, Zeke Yeager is another, Ymir (like the curse?) is a third; Erwin Smith comes up even more, but Historia Reiss is the one they mention most often. When you ask about her, they tell you that she was once the Queen of the walls and is now the Queen of the land across the sea, which they have since redubbed Haven (it was Marley before). She is perhaps the most powerful diplomat in the world, but apparently she still finds some time to visit you every once in a while. It saddens me that you didn't get to meet her this time around; hopefully, you will.
They've shown you pictures of Historia – photographs, they call them – and you can't help but laugh at how much she looks like Armin Arlelt. Armin has been your best friend since childhood, he says. He's also a Titan Shifter, but he gained his powers in 850 and thus has four more years to live after you're gone. It'll be weird, he tells you when you ask, but life is weird. In the meantime, he smiles at you and regales you with tales of stupidity from your childhood. He tells you that one day, you'd resolved to reach the outskirts of Paradis together, to wade through the ocean waters that so many former Wall citizens now take for granted. He tells you that you did it; you, him, and Mikasa.
Mikasa might confuse you a little bit at first. They say she's your favorite, but why was she the last to say hello? Sasha brings you breakfast the moment you wake up in a cold sweat. She rushes Armin, Jean, and even Floch into the room, and they all look more or less overjoyed to see you (mostly less; Jean isn't terribly enthusiastic and Floch looks irritated most of the time, but Armin makes up for them both). They tell you about everyone, and then you'll probably ask about Mikasa, and they might say something cryptic along the lines of, "oh, you should probably talk to her."
Whenever you find her, perhaps right away or perhaps later in the morning, she'll be crying, apparently as she has been since the night before. She's awfully gorgeous, even when she's crying. If you're the same as you were today or apparently have been since you met her at 9 years old, you'll have an overwhelming desire to wipe away her tears.
Today, you don't know if wiping them is the right choice or not. At least today, when you did that, she cried a heck of a lot harder for a really long time. But when she stops, and finally, finally smiles at you, it will feel like home.
The day will pass in the blink of an eye. Maybe you'll wrestle with Sasha for the last piece of meat; maybe you'll shoot the shit with Jean as you finally work a smile out of his grumpy horse face as you get him to recount the good old days; maybe you'll even get Floch to like you a little more (although today, you weren't able to); maybe Armin will show you his old books about the ocean, and he'll pull out one precious photograph of you, him, and Mikasa frolicking in the ocean (maybe, if you have more foresight than you did today, ask them to take you there!).
But I, Eren Yeager, would like to think that the day always ends the same way.
She'll lay beside you in the bed, the same way she used to, and it will feel like home.
Because as you wake up every day, just as you have woken up every day for the past however many years with a blank slate of memory, she'll be there.
We don't really understand time, she'll say.
And you won't really understand what that means, but somehow you'll understand what she's saying.
I missed you, she'll say.
You don't really know if you have to remember her to have missed her, but somehow you know that you missed her too.
I love you, she'll say as she kisses you on the forehead.
You know for a fact that you love her too. You hold her closer.
You can't wait to see her in the morning.