It is in his third year at Hogwarts that Teddy Lupin finds the Mirror of Erised.
He does not know what it is at first. His godfather has never told him about it – probably because he knows from experience what will follow.
He stumbles into an empty classroom late at night. Along with his mother's Metamorphmagus abilities, Teddy has inherited a knack of tripping over his own feet. He only knows his way around the school without attracting attention because along with his mother's ability to morph came his father's proclivity for wandering every corridor of the school illicitly.
He sees a lone mirror, with letters written across the top – Erised stra ehru oyt ube cafru oyt on wohsi. He does not know its purpose, or meaning. Curiosity overcomes him. He looks.
Two people stare back from next to his own reflection. A woman with a wide smile and pink hair; a man with a scarred face from which shines a more hidden smile.
He does not remember them. Not a single memory of them. He knows them only from the photos he has and the stories that have been told. Of bravery, of loyalty, and – most of all – of love.
He sees himself with his family, a desire held by so many before him, yet that has never been fulfilled. He sees his parents alive and with him.
He is found one night by Professor McGonagall, who can tell that he already knows of its purpose. She can no longer walk without a stick – even the magical must eventually succumb to old age – but she sits with him, telling stories about his parents. Of bravery, and loyalty, and love.
After that, he does not seek it out again. He knows that the desire will consume him. And he knows that, no matter how many tears are shed or how much sleep is lost, the mirror will only ever show a desire for him, and never what will truly be there.