They started sleeping together long time ago, Kanda thought it was on the first mission they got when Lavi joined the Order. He couldn't remember the exact reason, maybe it was because they had to share a bed, maybe one of them had nightmares. It didn't matter anymore.
Whenever Komui sent them on a mission or both happened to be in the Headquarters at the same time, Lavi would come to his bed and slip under the covers. At first he was a bit weirded out, especially when in the morning Lavi's arms were tangled around him and he could feel the other's steady breath on the back of his neck. It reminded him of someone he was friends with in the past.

"Sorry, Yuu! It's so cold, I'm freezing! Wanna cuddle?"
"Go to sleep or fuck off."
"Aww, so harsh! But seriously, will ya mind if I get a bit closer?"
"If it makes you shut up, go ahead."

After some time Kanda was used to the boy's warm presence next to him. When they woke up in the morning, Lavi would give him a sheepish smile and ask to wake him up in "a few minutes". Kanda gave in, knowing that Bookman made him do all of the paperwork till midnight.
Not so long after that, Kanda often found himself embracing the other boy, possesively holding him in his own arms. He didn't know why, but the warmth of Lavi's body let him feel at home. He'd often stay up all night long, listening to the sound of redhead's breathing. It calmed him down, soothed the pain in his head and numbness in his heart.

"I know you hate me but why are you trying to strangle me in my sleep, Yuu-chan?"
"I'm not strangling you, stupid!"
"So... Are ya hugging me?"
"No."
"Y' are!"
"Idiot, go back to sleep."

It was just after their mission in Edo when Lavi often woke both of them up with his screams. The horrible nightmares came almost every night and Kanda couldn't help but grip on the boy's shirt and try to calm him down.

"'s no big deal. Just a few akumas running after me."

Lavi tried to pretend everything was fine but the dark circle under his eye told Kanda that it was worse than bad. He could hear that the redhead's voice was raspy from screaming. He could also see how Lavi stayed up for longer than usual, not wanting to sleep.
It wasn't untill Bookman decided to keep Lavi away from everyone when Kanda finally got to know about his secret.

"I'm not supposed to care about anyone in here, Kanda." Lavi told him, his jade eye looking down at his own hands.
"So?" He asked, not knowing what the other meant. "It's not like I'm some teenag..."
"No, stop. Ya don't get it." The redhead held his hands in front of him, stopping Kanda from talking. "'m a Bookman. Bookmen don't need a heart. We musn't get attached. We musn't get involved. We can't care..."
"I don't expect you to care." Kanda cut his friend's words off.
Lavi's gaze shot up. He continued to blink rapitedly as the words sank into his mind, Kanda thought he looked extremely stupid.
"Ya okay with it?" The Bookman's apprentice asked, the doubt strong in his tone.
"Just don't expect me to care either."
Lavi's genuine smile was worth every lie.

That night, after Lavi told him about all of his personas and life as a Bookman in training, Kanda clenched his arms tighter around him. He felt as if the boy's body wasn't able to stand the pression on his past 48 lives. As if he was going to be ripped out from the inside and all of the people he was, and at the same time was not, were going to break out and run away.
He understood him, through. He had memories of a person he was not and sometimes it was hard to stay sane. He'd always thought that Lavi was just an idiot, but in fact, Lavi was one of the few people that actually earned Kanda's respect.

When a sudden turn of the events brought back Alma, they were separated. Kanda didn't have time to think, everything was happening too fast, collapsing at him one second after another. Old regrets and fears laughing in his face made him vulnerable, he wished he could just close his eyes and drift away from mess.
Alma died soon after Allen had sent them to Martel and Kanda was left all alone with his nightmares.
And when sometimes, just sometimes, he'd stay up all night long, his arms would feel empty.

AN:

orz sorry if there's any typos or grammar mistakes!
*hidees in her corner*