In hindsight, it takes Sirius a lot longer to work it out than he'd care to admit.
They make it through first year without getting expelled (which Sirius thinks is against all odds) and after a summer where his parents, after the initial screaming, which works as well as it ever has, basically ignore him but Regulus follows him around like a lost puppy, Sirius is glad to be back. The first two months pass without incident – James insists that setting a small, easily put out fire does not constitute an incident – and though Remus still goes missing every so often, Sirius knows better than to ask him about it.
He always offers them a reason now, anyway. His mother's ill, often. He's sick, too. Something else with the family. He's provided a reason every time since that first one, when Sirius caught him sneaking back in.
James offers his sympathies every time and doesn't seem to think on it any. It's just the way things are, he says to Sirius one night, when Sirius can't sleep. Peter's snores are quiet but they fill the room well enough. It's just the four of them now and they've banded together, so it's odd when Remus isn't there, even if it is pretty often.
It's December when he realises. James is asleep, Peter too. Sirius has his arms crossed behind his head and the curtains around his bed are open – in case Remus does come back in the middle of the night. He doesn't always come back after one night; sometimes it's two or three, but Sirius is ever optimistic that he won't stay away at all.
He sighs, shifts onto his stomach and looks out of the window near his bed. It's a windy night – it's been threatening to snow all week, too, which James is inordinately pleased about – and the whistle of air through the trees is a little unsettling.
It's not too dark to see, however. The moon is full and large, illuminating the grounds set out below them. Sirius looks, although he has the layout committed to memory and when his neck starts to hurt, he rolls onto his back again-
And promptly sits bolt upright because how did he not get this before?!
He concedes, quickly, that most witches and wizards in the school wouldn't have worked it out, but then, there are a lot of muggle-borns and they don't study dark creatures until third year at least and-
Sirius swallows around whatever's stuck in his throat. He doesn't want to think it's fear, because that's just cruel, but his hands are a little clammy and his pulse is racing. His eyes find the sleeping forms of James and Peter in the darkness. Do they know? Peter, definitely not. He doesn't think James does either, because he thinks James would have told him already, tried to work out how to approach the situation.
He wants to wake James right now, to tell him that he thinks maybe, possibly he might be right- but he knows that isn't the right course of action, either.
Instead, he falls back down to his bed, his head hitting the pillow with a muffled thump.
Remus is a werewolf, Sirius is pretty sure. It would explain a lot of things – why he always looks like hell the next day, even if his mother is supposedly ill; why he's always going missing for at least one night – and Sirius is positive it's always on the full moon, though he thinks he'll have to keep track, have to check-
He isn't sure when he falls asleep, finally, but when he wakes up the room is bright and James and Peter have gone. It's a Sunday, which means they'll be making trouble Merlin knows where and Sirius sits up, wiping his groggy eyes.
Movement alerts him to Remus and he smiles at the other boy sleepily before his thoughts from last night catch up to him. Remus looks like crap, as usual – but this time, Sirius thinks he can see a red mark, like a scratch, poking out from underneath his mussed collar.
Remus shrugs the shirt up properly when he notices where Sirius is staring and his smile doesn't quite reach his eyes. Sirius swears he's planned to do more research, to approach this carefully, cautiously (which he knows isn't like him, but this is Remus, who some days fights James in Sirius' head for the spot of best friend), but when Remus opens his mouth to say something, Sirius blurts out, "I know."
Remus eyebrows shoot up so fast, they almost reach his hairline. He tries to cover his surprise though, turning his expression carefully blank. "Know what?" he asks.
"I know, Remus," Sirius says. He doesn't think he's been this serious about anything in his life and it's kind of scary, but he also needs Remus to know that he knows so that he doesn't do something to mess it all up.
"Well. I." Remus paces back and forth for a moment and when he reaches to run a hand through his hair, Sirius sees that it's shaking. He thinks he can see the pulse racing in Remus' throat, too, even from this distance. He wonders whether he should get out of bed and then isn't sure what to do if he does.
"I guess, I'll-" Remus closes his mouth, then reaches for his books, which are piled high next to his bed. "I'll stay somewhere else, at least. I'll have to- I mean, I knew this was a bad idea-"
Sirius does jump out of bed at that, crossing the room quickly to tug the books out of Remus' hands. Remus looks at him, his expression utterly defeated. "I don't want you to go anywhere," Sirius says, quietly. He knows it's true. They've been together in this room for a full year before this.
Besides, it's Remus. Remus is patient and gentle and, above all, entirely in control. Sirius knows that it's probably different when the wolf comes out in him, but clearly he has something worked out for that situation.
Remus looks surprised, but there's some disbelief in his eyes. He shakes his head. "No. It's only a matter of time."
"I won't tell anyone," Sirius says - pleads. "Just stay. I won't even tell James, I promise. You can't leave us."
He's not sure which sentence tips Remus over the edge, but suddenly his eyes are welling with tears and Sirius drops the books on Remus' bed so that he can hug him. It should be kind of awkward, he thinks, but then he can't think of any other action that wouldn't be, so he holds on until Remus sniffs a couple of times and pulls back. "You don't care?" Remus asks.
"No," Sirius says, because there's the initial confusion at figuring out that one of your best mates is a werewolf and then there's the aftermath. Plus, now it means he knows what to look out for.
He smiles at Remus and, tentatively, Remus returns it. They both put the books back and Sirius showers and changes as Remus settles down for a nap. Before he heads out to find James and Peter, Sirius looks at the sleeping boy for a long moment. Whatever feeling it is that settles in his chest, it isn't disgust, or fear. He thinks, maybe, it's some form of relief.
Sirius isn't quite sure when James figures it out. He certainly doesn't tell him, but James is much smarter than even he gives himself credit for, sometimes.
Either way, he doesn't mention it. Sirius jokes with Remus about it occasionally, when he's sure the other two aren't anywhere nearby. Eventually, Remus seems at ease with the idea of someone else knowing.
Two months after Sirius' own revelation, Remus returns in the evening instead of the morning. He looks particularly bad this time - Sirius winces at the pale, haggard look on his face. He smiles weakly when he notices Sirius looking.
James looks up, looks between the two of them and then returns to reading through his Charms book, smirking a little. Sirius frowns but doesn't say anything. He knows if James is reading something he's trying to come up with a good idea for another prank, anyway. He probably hasn't even noticed.
Peter is still struggling with his Astronomy homework. "Are you alright, Remus?" he asks as Remus lowers himself to the floor next to Sirius and Remus only nods.
Peter frowns, narrowing his eyes - then, to Sirius' dawning horror, he glances between the parchment he's been working on all day, to the sky outside, then back at Remus.
Sirius thinks Remus realises what's going to happen only about a half-second after he does and out of the corner of his eye, he sees Remus' spine go rigid.
It's James who speaks, though. "It's not like it changes anything, does it, Pete?"
Peter blinks owlishly, glancing between James, who has a deceptively mild expression on his face, and Remus, who looks like he'd run if he thought he could.
Peter shakes his head and then laughs to himself. It's a little weak and a little high, but it's enough to relax the atmosphere a little - enough for James to return his eyes to his book, at least.
Sirius gapes at his best friend and then he's laughing himself, loud and long. James' lips twitch and he hears Remus join in next to him.
It doesn't change a thing.