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Harry turns eleven and doesn't notice Canis Major forming star by star on his back. It's only later, after he properly learns about soulmarks at Hogwarts, that he searches for one on his skin. He breathes a sigh of relief when he finds it. In this one thing, he is no different from the other Hogwarts students. Relief turns to amazement when he sees the way it glows faintly in the dark. One star in particular shines more brightly than the others.
The very next day, Harry checks a book on constellations out from the library and finds his soulmark inside. Canis Major, whose brightest star is Sirius.
Sirius, Harry thinks to himself, and smiles. Sirius.
He's given less cause to smile once he finds that the only people named Sirius are dead or in prison for the murder of twelve muggles, after having betrayed the Potters to the Dark Lord. He's grateful that soulmarks are a private thing and that Ron is only barely scraping by in Astronomy. Ron says nothing if he ever sees Harry changing and Harry says nothing in turn. His biggest secret is that he still admires the mark sometimes in the dark, looking back at a mirror to see the shine of his stars.
A few years later, Sirius is no longer quite as awful as the newspapers made him out to be, and Harry's mark warms when Sirius hugs him. Harry looks to Sirius to see the realization in his eyes—Sirius's own mark must have reacted in kind—but Sirius is already patting him on the shoulder and telling him he'll write.
There is no talk of soulmates in Sirius's letters. Harry lets it go. There is sabotage and attempted murder and teenage hormones to be dealt with; maybe it's a good thing, that his soulmate doesn't want to acknowledge him. Voldemort is more important than whether Sirius is put off by Harry's scrawny arms.
It's only later, during winter break of his sixth year, that Harry overhears a conversation between Sirius and Remus. He could claim that he hadn't meant to eavesdrop, but that's never true when it involves a mystery or his soulmate. Sirius is the most interesting thing in the lousy old Grimmauld Place.
Harry's slumped over on the couch, eyes closed, textbook open on his chest, while he listens to Sirius and Remus talk at the other end of the sitting room. It's all about Remus and Tonks—boring, Harry thinks—until it suddenly isn't, and Remus asks, "Haven't you ever felt your mark hum?"
Every time you hug me, Harry thinks. He keeps his eyes closed.
Sirius's voice is so quiet that Harry can barely hear it. "You know I haven't. I'm not meant for that kind of thing. Soulmates, true love, all that rubbish. I've never felt anything." He stops for a moment, then adds, "Fuck it, I doubt I ever will. I've had trouble with my magic ever since Azkaban. It's not as responsive as it used to be."
"All you need is time," Remus assures him.
They leave for another room, leaving Harry to his own thoughts. The very next day, Harry finds an excuse to take Sirius's hand, claiming interest in the faint scar that ran across the base of his thumb. While the story about how Sirius had needed to get it reattached as a kid is funny, Sirius gives no reaction of feeling anything amiss. Harry thought that maybe Sirius's mark would react to prolonged contact, but that doesn’t seem to be the case. He holds on for as long as can be considered normal, tracing the scar and turning Sirius's palm over a few times.
In the coming years, Harry grows used to touching Sirius absently in innocent ways. Or, ways that could be construed as innocent. Part of it is the hope that someday Sirius's mark would react. The rest is simply that Harry seeks comfort from his soulmate, unknowing and one-sided that their connection is. And during war, he could use the comfort. After the war, there is grief and relief and habit that keeps Harry tactile.
Sirius never calls him out on it. Harry thinks that he doesn’t notice it; that maybe despite Harry's affection, his growing feelings, nothing can develop between them as long as Sirius's end is closed off. He's never even seen his godfather date. For all that Harry knows, Sirius has no interest in love, true or otherwise.
Those thoughts send a pang through his heart. He reaches for Sirius absently, resting his hand against Sirius's upper arm as he joins Sirius in looking inside the cooling cupboard. It's going to be another takeout day; Harry can't be fussed after a long day of auror work and Sirius is rarely interested in cooking something other than breakfast food. Harry's soulmate mark hums as per usual and Harry lets himself enjoy the small moment.
Beside him, Sirius lets out a startled breath.
Harry slips his hand off, hoping that Sirius hadn't suddenly realized all of his unnecessary touches. He would have to be more subtle about it, but Harry doesn't do well with subtle. "Leaky takeout? I could do with pot pie."
The cooling cupboard closes without Sirius holding it open. When Harry looks his way, he's staring at Harry with a breathless expression, and the next thing Harry knows, Sirius's hand circles around his wrist. "Were you ever going to tell me?"
Harry wet his lips unconsciously. "It seemed a little rude to spring it on you when you couldn't feel it. Didn't feel right."
Sirius's grip doesn't loosen from his hold; instead, his other hand finds its way to Harry. "We're going to have a conversation about acceptable secrets to keep from your godfather."
"Sounds like a fun conversation," Harry says, taking a half step closer. "Is it going to be now or—"
"Later. I need to find your mark, first."
Sirius wastes no time in kissing him. Harry could tell him that his mark doesn't rest within his mouth, but that would be ruining the fun. He kisses Sirius back with all the ardor that has built in his chest along the years. Later, he'll ask about just how long Sirius has felt the same, and he'll find his own mark somewhere on Sirius's body. But for now, all that matters is the way Sirius presses him against the cupboard and the way the constellation on his back sings in response.