[Illusion is the first of all pleasures]
The cloak James presents them with opens up new doors for exploring Hogwarts. It's a tight fit, squeezing four boys under one cloak, but they manage it. They zip down the halls long past curfew and take advantage of their newfound freedom to make adjustments to the Marauder's map, which is nearing completion. They play hide-and-seek in the darkness like children, something Peter excels at, and one of the few games James and Sirius can prod Remus into participating in.
It's how Remus stumbles into the Room of Requirement one night, and finds himself drawn to a mirror in the corner. He reads its inscription aloud, fumbling with the odd-sounding words but not knowing what they mean. And he thinks nothing of it at first, until—
His scars are gone. He looks calmer, brighter than usual. Free of tired lines beneath his eyes and a smile a mile wide. His clothes are new and... It's him and Sirius, and Sirius' arms are around him; he's nestling his face playfully into Remus' hair, grinning in a way that makes Remus' face warm and his chest flutter. They stand under a full moon and yet Remus is human, so perfectly human.
Somewhere in that image, the longer he looks, are his friends, his family, laughing and smiling and happy. It shows everything perfect and right in the world and he can't look away.
It's why he returns the next night, alone. And the night after that, and the night after that. Marveling at the wonders the mirror presents to him.
He understands what he sees is nothing more than mere illusion and yet, he wonders. He wishes. He hopes.
And when Sirius finds him one night with the now-completed Marauder's map, Remus leaps to his feet and tries to throw the tarp over the mirror out of fear Sirius will somehow see exactly what Remus sees, and the humiliation he will feel will be the end of him.
"Is this where you've been hiding the last few nights?" Sirius asks. Remus drops the tarp uselessly about his feet as his friend slides up beside him. "Really, Moony, what's so interesting about a dusty piece of junk?"
Remus bites his lip as Sirius stares into the mirror. Sirius squints, blinks, raises his eyebrows, tilts his head like an inquisitive dog trying to figure something out. Remus thinks he shouldn't ask, but he can't help it. "What do you see?"
Although he doesn't expect an answer, Sirius's voice is soft, almost thoughtful. "Me and you, Remus. Just me and you."