Manfred Grieg

Manfred Grieg, also known as The Man in the Mirror, was long ago trapped 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 within Castle Extremis. He revealed himself to The Doctor and Martha, when he realised that something was going on inside the Mirror. (NSA, Martha in the Mirror)