Note: This was my quickly written contribution to Eremin week on Tumblr. I have this headcanon that Eren has these really vivid nightmares after he's been in Titan form, and Armin is the only one who can keep him from having these nightmares and likes to sing him lullabies and stuff while petting his hair until Eren falls asleep.
So yeah, in other words, pretty much just fluff.
Armin was curled against him, back pressed to Eren's chest. He found he slept better with Armin next to him, especially after he'd spent any amount of time in his Titan form. Despite his small size, Armin was solid, a constant presence. And more importantly, a reminder that this wasn't a dream.
Sleep could be a tenuous thing for a while whenever he came out of the Titan body. Dreams he experienced there blurred with dreams he had sleeping in his own bed, or he had nightmares so terrible he woke up screaming. Dreams of attacking the town, or Mikasa…or Armin.
That dream had started their current sleeping arrangement. Eren could still hear Armin's screams as he put him in his mouth, could still hear the crunch of bones and gush of warm blood in his mouth when he woke. Thrashing and screaming, he'd demanded to see Armin, to know that he was alive and safe, and then had refused to let him go. That nightmare returned every night until Armin ended up sleeping with him every night. The nightmares slowed down after that.
Eren's arm curled tighter around Armin's waist, and he placed a gentle kiss on his lover's shoulder. Armin meant more to him than anyone; if something ever happened to him…
"Eren?" Armin grumbled sleepily.
"Go back to sleep," he answered, lips still against his shoulder.
Instead of going back to sleep, Armin turned in his arms and shifted so that their noses were touching. His eyes were barely open. "You should be asleep, too."
"I can't." He didn't want another nightmare.
"Eren…"
"I'll be fine. You need your…" he was cut off when Armin closed what little distance remained between them and kissed him softly. Eren held him closer, not wanting to let him go.
"You need it more." Armin's fingers were gently combing through his hair, and Eren would deny it if anyone ever asked, but when Armin did that it made him purr. Eren's eyes were already getting heavy.
"Armin…" he protested, but Armin kissed him again, and he didn't have it in him to argue further.
"Let me help you, Eren. For all the times you've helped me."
Knowing Armin would do it anyway, Eren nodded. Armin's signing wasn't loud at all, barely above a murmur against his lips, but it was warm, comforting, safe. His voice chased away all the nightmares stalking the edges of Eren's consciousness. Eren held onto Armin and his voice like a lifeline. Just before he drifted into sleep, Eren felt a soft kiss on his cheek
That night he dreamed of Armin, of a place by the ocean where they could sit on the beach wrapped in blankets and watch the waves crashed against the shore. Of Armin's voice still singing soothing songs in his ear and stolen kisses between verses.
He dreamed of home.