In the old time, no one gave much thought to their own face. You knew your hands, your arms, the visible parts. Your face was what others reported it to be: your mother said you had your grandfather's eyes, your neighbor said you were beginning to look like yourself. This was how everyone knew themselves, more or less.
Then came the polished metal, and people could see themselves directly for the first time, and many of them were startled, because what they saw did not match the collection. The collection had been generous in ways the metal was not. Some were glad for the clarity. Others found they preferred the version assembled from others' accounts and mistrusted the glass.
One woman saw nothing when she looked at the metal. It showed the room, showed whoever stood nearby, but when she looked for herself she found only the wall behind her. She was not alarmed. She had always assembled herself this way; she simply had no other option. She kept asking. She kept writing down what people said. She became the most precisely documented person of her generation.
She died at eighty having assembled the most accurate self-description anyone had produced. It was entirely secondhand. It was more reliable for that, because she had no reason to flatter herself and no investment in any particular version. She was whoever the people who looked at her saw, and she had learned to weight their observations and discount their biases, and the result was something closer to the truth than most people arrived at through the mirror.
There have always been people like her. They find each other eventually, and compare notes, and share sources. They are better documented than the rest of us. They have to be.