He is the only one that volunteers to watch over the crystal.
At first, it's awkward. Sitting alone in the cave, staring up at her too still form. He doesn't approach the crystal until almost two months later, scared that if he touched it the stone would shatter with her in it.
He finds that the crystal is smooth, cool, and unyielding, much like Annie's personality. He takes his time tracing over every feature, from the sharp curve of her jaw to the gentle dip of her lips. He imagines what it would be like to press her lips against his own. Would they be cold and hard or soft and warm? His fingers linger on the pouty bottom lip.
He wishes he could make her smile. She's so pretty when she smiles.
His fingers trace down the slope of her neck and to the curve of her shoulders. His eyes are drawn downwards but he hastily looks away. It feels wrong to look below her face. So he goes back to tracing her face and hair until he's about to drop from exhaustion. When he can no longer stand, he retreats back to the small bed in the corner and settles down, his eyes focused on her until sleep over takes him.
After almost six months of almost continuous watch, Armin conjures up the courage to trace Annie's figure. He starts on her arms, running his hands down them, touching his fingertips to the long digits partially obscured by the thick crystal. His hands slide down her hips until her knees, when he can no longer see those long legs.
Trembling, he pushes his hands up her thighs and over her hips, closing his eyes to imagine the soft curves beneath his fingers. His hands curl up as he reaches her breasts. His palms flatten over her chest and his breath catches in his throat as he thinks about kneading and caressing them. How Annie would moan under his touches, begging for more.
He would be helpless to disobey.
When he goes to bed he's wide awake, desire still pumping through him. Every time he closes his eyes he imagines her above him, grinding into him, riding him desperately. He bets that her face becomes flushed and she becomes an incoherent mess.
Armin gasps at the touch of his hand on his own cock, stroking up and down, up and down. He closes his eyes again, the image of Annie on her knees, head between his legs as she works him with her hand and mouth. He muffles the sound of pleasure with a fist shoved into his mouth. Teasing his head, he pretends that it's her tongue stroking along his slit, spreading the white liquid around.
He calls out her name as his seed splatters against his stomach. It echoes in the cave and his entire body flushes with embarrassment. He cleans himself up with the blanket, refusing to look at the crystal, knowing that Annie has witnessed the act.
For the next few weeks he lets Hanji and others watch over the crystal. He can't bear to face the shame, despite the fact that when he's alone in his room at night he comes to her name, over and over again.
When he comes back down to the crystal, his first words are, "I'm sorry Annie." He kneels down before her and stares at the ground, as if waiting for judgement.
He receives none.
Finally he is able to ease back into their living arrangement. He begins to talk to her of the surface world doings. How they're advancing on the titans, discovering unimaginable secrets. "Eren's become more level headed during battles. We're closer to defeating the titans more than ever. I'll be able to see the outside world finally!"
Armin eventually brings his book of the outside world downstairs and he leans up against the crystal and reads to Annie, pointing out all the different pictures. "Imagine a field of sand or ice. The only time I've seen snow is during the winter, and even then it isn't that thick. To think of places that the snow is so deep that it comes up to my head! I mean, it's not like I'm that tall to begin with." He chuckles.
He spends countless nights like this, reading to her before eventually falling asleep against her crystal. It becomes a habit. When he runs out of things to reads to her, he simply talks. Talks about his childhood, about his hopes for the future.
About how he always had a crush on her and how he regretted not realizing his feelings soon enough.
Even if it does make him feel dirty and like a traitor to have such strong feelings for the female titan.
The days pass into months, and the months into years. Armin leaves to go on trips with the scouting legion, always hurrying back so he can get to Annie. Eren and Mikasa notice but they say nothing. It would be useless to discourage him. Besides, they attribute it to a mental breakdown that Armin was bound to have sooner or later.
By the time Armin turns twenty-one, so many things have changed. Titans are no longer a threat - occasionally a rogue will be dispatched with ease but people are more willing to venture outside the walls for the first time in a century. This leads to the discovery of other human settlements, resulting in increased trade and wealth.
Armin himself has changed. No longer short, he beats Jean and Eren out by a few inches and having gotten control of his new extended limbs, he becomes one of the best fighters and tacticians of his generation. He is world weary though, having traveled but never found anything to satisfy his desires.
He always finds his way back to the cave, now abandoned, and back to Annie.
One day he stands there, tall enough to look at her face to face without the aid of a stool. He cups her face, mesmerized by the size of his big palm curving around the smallness that is her jaw. He thinks of how she used to look so big and intimidating.
Now she looks like a fragile flower, easily snapped in the wind.
He retreats to this cave, keeping in contact with others but preferring the torchlight to sunlight. He feels guilt for all the time he left her unguarded and he promises that he'll never do it again.
It's drawing close to evening as Armin sits at his desk, penning a letter to Eren and Mikasa, reminiscing about times gone past. He misses his friends but he knows he has no place in their busy world.
At first, he thinks the cracking sound is the desk under the weight of his arms and he eases off, staring at the wood suspiciously. The cracking sounds again, louder and this time behind him. He whips out of his chair, pulling out a small dagger as he expects some intruder to come leaping out. He backs up towards the crystal, intent on protecting Annie with his life. His love for her has only grown during her captive years and he will willingly die even if she never knows it.
Something hits his head and he looks up and behind. The crystal is starting to crack. He can see a fine seam running along the top, spiderweb cracks darting along. He steps back as the crystal begins to gently shake, the cracks growing larger and louder. His eyes widen and he ducks for cover right as the crystal explodes from the inside pressure. He is slammed up against the wall, shards cutting into his skin as steam becomes so thick he almost chokes.
When the steam clears he lifts his head, coughing lightly and dusting crystal bits from his hair, ignoring the blood seeping down his skin. Annie is on her hands and knees, breathing heavily. Armin isn't sure if this is a dream or nightmare and he just sits there for a moment, shock on his face. Annie slowly lifts herself and looks around, eyes wide. They land on Armin and silence drifts between them.
Finally Armin stands up with a smile breaking out across his face. He stumbles over to Annie and grabs her hands and pulls her up into his embrace, relishing in how soft she feels in his arms. He leans down and with no self control, he captures her lips. She's too stunned to kiss back but he's okay with that. Slowly he pulls away and whispers,