Remus's mother calls him "perceptive," which he believes is just a nice way of saying "freakish." It is a weirdness that has nothing to do with his "Furry Little Problem"—haha, James, yes, very clever—and everything to do with his cursed mind.
He thinks about everything. He can't help it. He tries hard, so very hard, to imitate Sirius and James and Peter and just be, but he can't. While they laugh and frolic and exist, he analyzes and watches and worries. Every word, every gesture, every look he observes is seared into his brain, filed away for future reference that he doesn't want to have.
Even when he is among them, he is aloof from them. It's like he's surrounded by a bubble, invisible yet impassable, that allows him tantalizing glimpses and occasional forays into the world of normal boys, but at the end of the day, he's still trapped in an iridescent prison.
Sometimes when he lies awake at night, over-analyzing as usual, he wonders if perhaps his mind works the way it does because of his condition, but then the paranoid part chimes in and says he's just trying to blame all his problems on the moon.
Sirius and James tease him now and then, all in fun, about his peculiarity, and he laughs with them and pretends it doesn't slice through skin, blood, and bone to the very center of him. Even Peter joins in, and Remus's misery deepens. If Peter is in a position to make fun of him, he is truly doomed.
Then he thinks of Peter, and—though he feels ashamed to admit it to himself—he is alarmed at how similar they sort of are, though no one would guess it. Peter yearns to fit in with the three of them, trots around on their heels trying desperately to win a single charming smile or precious laugh. Everyone sees him, and mocks or pities him as they please. What no one sees is Remus, who yearns for something similar. Each day he stands with his best friends, who shine brighter than the sun, and people assume that he's shining, too.
But really, it all comes down to the moon again—his radiance is only borrowed.
There are times when Remus is almost—almost, but not quite—grateful for his position. He sees what others don't, and he supposes that's all his mother means when she praises his "perceptiveness." For instance, he sees the way James pines for Lily, doggedly pursuing her even though she's hexed him more times than Remus can count. And he sees also the expression that sometimes crosses Sirius's face—it's really remarkable, the range of emotion that he can convey with the slightest twist of his lip, tilt of his chin—as James pleads with her for just one date. Remus thinks it's noble of Sirius to stand gracefully aside, and he wonders how difficult it is because Master Black is well used to getting what he wants.
Of course, half the boys in school are after Lily, but none of them are quite like James or Sirius. (Or for that matter, one Severus Snape, but the thought of him causes an unpleasant lurch in Remus's stomach so that his mind snaps shut like the jaws of a…No, see there? Stop it!)
And Lily…well, Remus finds it difficult to analyze when he thinks of Lily directly, because his mouth goes dry and his palms sweaty and his heart rate jumps, so he carefully skirts around Miss Evans. It's much like looking at something out of the corner of your eye—you're aware of it, but you know if you look at it directly, it will disappear.
Lily knows all about James's infatuation, of course, but sometimes the oddest look crosses her face and her eyes get a bit greener (if that's even possible) and Remus wonders fearfully if she might harbor suspicions about Sirius as well—or worse, Remus himself. She is so terribly, terribly clever, after all. That's part of what draws him to her, but it's nowhere near the largest part.
Lily is kind to him, but it's more than that. She comprehends him in a way that his best mates do not, and she can convey an entire world just by meeting his eyes. They study together sometimes, and over parchments in the library, with the smell of old books swirling in their nostrils, they pour out their secrets to one another. He marvels at the fact that such perfection could not bar insecurities, and the knowledge endears her to him further.
"You're a true friend, Remus," she whispers, leaning forward to hug him, and he laments that that's all they'll ever be, for he sees with a heavy heart the way she looks at James as he's pelting away from her down the corridor pursued by a flock of angry birds.
But it is at least a small comfort that Remus will always have Lily to confide in, even if he will not have her heart to keep. Perhaps it's better that way, anyhow—Remus wouldn't know how to hold it without bruising or letting it slip from his shaking fingers. James is really much better suited to that sort of thing.
So when the wedding day dawns and he's watching Sirius watching James watching Lily, there's only the tiniest measure of despair lurking in the darkest corner of his heart. And when her green eyes stray over the audience to collide with his, he offers her an understanding smile which she returns in kind.
At that moment and for years afterward, he grieves because he can only watch his world, not live in it, and when the bright lights are extinguished, he doesn't even want to do either. But redemption finds him, unwilling, and when the time finally comes to prove that he, too, can live, fate ironically decides that with his newfound knowledge, he must die fighting for the chance.