Manfred Grieg

Manfred Grieg, also known as the Man in the Mirror, had been trapped long before inside The Mortal Mirror and was cursed with a perpetual body of glass. He kept a glass diary, which seemed to explain the past, present and future: everything that happened in Castle Extremis. He revealed himself to the Tenth Doctor and Martha, when he realised that something was going on inside the Mirror. (NSA:  Martha in the Mirror)