A lonely young boy, Walter Simeon encountered alien snow that was "slightly telepathic" while making a snowman, and unwittingly began pouring his darkest thoughts into it, causing the snow to develop a form of sentience. Simeon grew up and became a scientist, with his accidental creation (later known as the Great Intelligence) as an advisor — Simeon believed that the snow being's orders were its own rather than a reflection of himself, which, in truth, became closer and closer to the truth as the Intelligence grew in power and intellect. Simeon and the Intelligence began to go about building an army of ice for himself to be in power. To this end he established the Great Intelligence Institute. (TV: The Snowmen)
Facing 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. (TV: The Snowmen)
Although Simeon's physical body had died, the Great Intelligence used his image as an avatar to communicate with Rosemary Kizlet via a large wall-mounted video screen at the Shard. (TV: The Bells of Saint John) The Great Intelligence used Simeon's form as its appearance as well. (TV: The Name of the Doctor) In fact, the reports of "Simeon" being with the Whisper Men cast the sightings into doubt, as Simeon was known to have died in 1892. (PROSE: The Secret Lives of Monsters)
The Intelligence appeared 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. (TV: The Name of the Doctor)
The Intelligence used comparable avatars on other occasions, such as when he escaped into 1937, appearing before three boys in Draynes Wood, Cornwall (including an eight-year old Alistair Lethbridge-Stewart), and later in 1969 for a final confrontation with the adult Colonel Lethbridge-Stewart. (PROSE: The Forgotten Son)
Walter Simeon was a cold hearted sociopath, and, to the utmost represented part of his personality, a misanthrope. As a child, he believed that other children were "silly" and didn't want to play with them. His mother commented that his solitary nature was unhealthy, and since Simeon never even tried to emotionally connect with other people he grew into a heartless man who showed very little emotion. He allowed his Snowmen to eat a group of homeless people who had been building them without any concern for their lives, coldly stating, "I said I'd feed you. I didn't say who to." An intelligent and very power hungry man, Simeon was determined to take over the world with his ice army, although for most of his life he thought he was obeying the orders of the Great Intelligence, not realising that it was a reflection of himself. The one time he did show emotion was when he realised who and what the Great Intelligence was, seeming shocked and slightly fearful of the entity he had served for almost his entire life. Nonetheless, this did not stop him from trying to dominate the world, which he and the Great Intelligence had been working towards for fifty years.
Simeon was also shown to be arrogant and casually informed Vastra and Jenny Flint of his plan, believing that there was nothing they could do to stop it. When Vastra said she knew someone who could stop him, Simeon merely responded, "I look forward to meeting him", not taking the Silurian's words seriously. He was also indifferent when he lifted Vastra's veil and saw she was a Silurian. (TV: The Snowmen)