Doctor Simeon

Dr Walter Simeon was a man associated with the Great Intelligence, a faceless entity who went on to use his image as a body long after Simeon's death.

Walter Simeon found the Great Intelligence as a young boy, and unwittingly began pouring his darkest thoughts into it. Believing its orders to be its own rather than a reflection of himself, Simeon began to go about building an army of ice for himself to be in power. To this end he established the Great Intelligence Institute and sought to bring the Ice Governess to the Intelligence, which brought him up against the Doctor.

When he confronted him at the Institute, the Doctor showed him how the Intelligence was nothing more than a reflection of Simeon himself rather than a separate entity. Undaunted by this revelation, Simeon sought to get the remains of the Governess, but the Doctor tricked him into being bitten by a memory worm. The memories of his adult life were erased, but the Intelligence had by this time gained enough independence to survive and fill in the void left and take over Simeon's body. After a brief scuffle with the Doctor, whatever was left of Simeon died when the Intelligence was defeated and relinquished control of his body.

Although Simeon's physical body had died, the Great Intelligence used his image as an avatar to communicate with Miss Kizlet via a large wall-mounted video screen at the Shard. The Great Intelligence would later appear in the form of Simeon on Trenzalore accompanied by the Whisper Men, who wore the same attire as Simeon, the reason for this was revealed when he destroyed his own body and immediately after manifested in a Whisper Man whose blank face turned into that of Simeon's.

_{Biography from the TARDIS Data Core article, licensed under CC-BY-SA}