Albus was the first to stumble upon it. He was a first year, exploring the school looking for the Room of Requirement, the only place he felt safe when he was feeling down, which was a lot lately. He was still upset about his sorting. His mother had cheered him up in her letters, sending words of encouragement and love, she and Albus were very close. But something was still nagging at him. He wished he was more like his brother. To him, James had always had his life together, and Albus wanted to be like that too. He wanted to be a brave Gryffindor quidditch player who made his family proud. But somehow he had turned out exactly the opposite.
So, naturally, when he found it, he thought he knew exactly what it would show him: an image of himself decked in Gryffindor red with his brother and quidditch team by his side, after scoring the winning goal.
He knew what it was from the moment he saw the corner of it peeking out from under a dusty, old, sheet. He was certain he knew what it would show him, so it was odd that he looked at all. But then again, it was the first time the Room of Requirement hadn't given him the cozy reading nook he had grown accustomed to. He never was one to believe in fate, but curiosity peaked his interest and he pulled the sheet off nevertheless.
Albus removed the sheet slowly for dramatic effect, closed his eyes, and braced himself for what he might see. When he opened his eyes he expected to see the emerald of his own eyes staring back, but he was met with only blackness. For a moment he thought it hadn't worked, that maybe it was just some old mirror, until he looked up, and saw his own face, only taller and more mature, above him. The black he had seen was the black of his robes resting, to his surprise, over a tie the same emerald color as his eyes. One of the older Albus's arms was slung over the shoulder of a boy he didn't yet know, the other clutching his Hogwarts diploma. He was smiling as his parents, now graying, who stood next to the slytherin boy and him, their faces alight with happiness.
Albus was shocked. More than shocked, really he was astonished. Was this really what he wanted? Or could the legendary Mirror of Erised be… wrong? As he stood there, gawking at the picture before him, as the minutes dragged past and the only sound was his ragged breathing echoing throughout the mostly vast room, he realized, it was what he wanted. The mirror wasn't showing him what house he should be sorted into, the hat had already done that. It was showing him what he wanted, and he found that what he wanted now, and had wanted all along, was happiness.