Since this was written before some of the revelations about Levi's background it's kinda AU at this point, so I hope you don't mind. Set relatively early in the manga.
He can't feel time passing, but he knows he has spent a short eternity in the bed. The only things to look at are the window and down at his lap and missing leg.
Out the window is unceasing 3DMG practice. Every time he looks out he can see them doing maneuvers, his maneuvers in the air. He looks down at his missing leg, instead, and the pain flares up, eating into his side, crawling up his body, as if it's not just his leg gone but half of him immersed in titan digestive fluids.
He looks out the window to forget the pain, and hears a clear voice congratulating, saying better than Levi! and he wants to tell them they're wrong, it's not perfect, even though it is. It's like seeing himself in a mirror multiplied by trainees.
An eternity passes and he fades into the silence, the pain. He will never leave this room, he thinks, and watches it shrink to the size of a coffin with just one window, through which he can watch the trainees in their airborne dance.
"Levi."
There is something else to look at. Erwin is beside him, alive and holding out his arms, and Levi is suddenly alive as well. There is something in his world besides visual and physical pain. He knows he shouldn't, but he reaches back anyway, lets his commander lift him up. It's like flying again, Erwin is so tall, and Levi feels relief relaxing his muscles, releasing the pain.
Levi's warm again now, and when he leans his head against the commander's chest, it's right, it's familiar, like he hasn't denied himself this since the day they met.
They are outside now, walking across a base almost as familiar to Levi as the maneuver gear he will never wear again. Above him soldiers flit on their wires, deep in their own worlds, and nobody looks at him. Levi closes his eyes against the sight.
When he opens them they're outside, walking through familiar streets that he hasn't seen in years and never wanted to see again.
"I don't want to be here, Erwin," he says, closing a fist in the fabric of his jacket, right by the Wings of Freedom. "Let's go away."
"Shh, Levi, just a bit longer."
Erwin isn't looking at him, his profile cold, though Levi doesn't understand, because his hands are so warm and safe.
The houses are rotting now, the people's eyes following them hungrily, and it's familiar, too familiar. Levi has been here before, in another lifetime. He's struggling now, trying to get away from arms that are no longer comforting, but wrap his body in steel bands, caging him. Buildings rise up on either side to meet at the top, blocking out the sky and leeching the world of light and color.
"Why are you bringing me back here?" he cries. "You said you'd take me away from this. You said I would fight for you."
Erwin doesn't answer, and doesn't even notice Levi's struggles.
"Please, don't take me back! Please, Erwin, haven't I been a good soldier? I did everything you asked of me! I'll do whatever you say, I gave you my life, I gave you my heart—"
They're standing in a stinking cul-de-sac; the same one Erwin had plucked Levi from so many years ago. Erwin's holding his hands out, pulling Levi away from his body though Levi's struggling to cling to him. But Erwin's too strong.
Gently, he lowers Levi to the filth which comes crawling to greet him, coating his body, filling his throat, muffling a last cry of Erwin's name.
Now Erwin's face is visible again, and he's wearing a gentle, fond smile. He reaches out to stroke Levi's hair.
"You were useful to humanity," he says.
He's walking away, and Levi's trying and trying to scream but he can't—
Levi jerks awake with a muffled cry. He's in his bed, in his room, his Wings of Freedom are intact on his cloak, visible in the dim light from the window where it's hanging off the back of his chair. His limbs are where they should be.
That fucking nightmare again.
Levi rolls to his feet and changes quickly, pulling on pants and buckling on his maneuver gear. The sky is mostly pitch black but there's a hint of dawn somewhere far off, and that's enough for him. He's probably slept at least four hours.
He strides down the silent hallways, empty but for the few people on watch at this hour. Nobody he encounters looks surprised to see him, but he avoids their glances, the feeling of eyes upon him making his stomach churn.
The practice grounds are empty and dark, but that's good, training in the dark is something most people don't do often enough. Levi starts warming up, even though the adrenaline in his veins is demanding that he get in the air, move, practice. Prepare against the day he's too slow, too weak, too distracted.
The day his usefulness ends.
It won't happen, he tells himself as he starts warming up in the air, doing the first set of swings and flips, no blades yet. The dream is nothing more than his subconscious fears that Erwin will abandon him. Erwin's not like that.
Only, he knows deep down that it's true, more true than anything else in his life right now.
Even if it doesn't play out quite that way, Levi knows too well that he's not like the others. Erwin is a strategist, Hange's a scientist. Levi is nothing but a fighter, and when he can fight no more, he will be retired and forgotten.
Erwin doesn't waste energy on sentimentality. He will not hesitate. Levi refuses to imagine that Erwin will miss him as anything other than a skilled pair of blades.
So he trains, even when he hurts and his eyes swim with exhaustion. Because if he can practice through exhaustion, perhaps he can kill titans through exhaustion, and it will not bring him down. He trains even when he's ill, in case he's ever ill on the day of battle.
Sometimes it feels like he's forever tired and aching, running on pure adrenaline for years on end. He wonders, as he twists his body through the air, what it would be like to fight clearheaded and bursting with energy. If he had time to sleep for a year, maybe he could find out.
The sky is no longer pitch-black when he lands, covered with a sheen of sweat, his nerves calm. He performed flawlessly, and can fall back into being Humanity's Strongest for one more day. He knows the maneuvers perfectly-
(-and knows how to botch them, should it become necessary to die rather than return broken)
Levi sheathes his blades and treks back to the barracks, his body heavy and awkward on the ground. Near the door he sees Erwin, who watches his approach with a soft expression. That softness will vanish in the course of the day, Levi knows.
Erwin shouldn't be here, yet he always comes.
"Again, Levi?"
They're still far from each other, but his voice carries in the silence. Levi shrugs a reply.
"I worry about you."
Good, Levi thinks, and now they are close enough to touch – which Erwin almost does, reaching callused fingers towards the dark circles under Levi's eyes. Levi twitches out of the way and Erwin drops his hand. He tries to meet Levi's eyes, and Levi knows that despite how cold Erwin seems, there are emotions in him still.
"I'm fine," Levi says, brushing him off. "Except for needing a shower."
"Levi…" Erwin takes a step closer and Levi pauses next to him, still refusing to look at his face. Erwin lays a hand on his shoulder. It should feel stifling, but it's warm and solid and—
-Too much like his dream. This comfort can turn to a cage in an instant.
"Yes, sir?" Levi says. He doesn't shrug his shoulders, but Erwin removes his hand nonetheless. Levi knows that Erwin would leap on the slightest hint of warmth from him, but will not speak his feelings unless Levi gives him a sign, any at all.
Which is lucky, because if Erwin bared his heart, Levi might not be able to refuse him.
"Nothing. Carry on."
This is the only thing Levi hoards for himself against that nearing day. Even if Erwin can live without him, he will leave Erwin with regret that he had never spoken. Erwin will never know if Levi would sleep with him if asked, the taste of Levi's mouth, or the heat of his skin.
When Levi is nothing more than forgotten trash, at least he'll know that Erwin will forever wonder what might have been.