Nardole (Shadow World)

Nardole was a companion of the Twelfth Doctor.

He existed in the Shadow World as a virtual replica of the original Nardole from the Doctor's World. Like the rest of the Shadow World, he was created by the Monks, so that they could plan the perfect invasion of Earth.

Nardole shared the history and personality of his non-virtual counterpart, only parting with that history shortly after the events aboard Chasm Forge. He was a perfect duplicate and, like the rest of the people living within the Shadow World, he believed himself to be real.

Theorising that he was simply a virtual projection, like the rest of his world, Nardole stepped beyond the light of the projectors. As he dissolved into pixels, he realised he was right: "I'm part of it! I'm part of the simulation! Bill, I'm not real!".

History
While exploring the Vatican, Nardole and Bill found a portal which took them to a room external to their simulated world, in which they found a series of portals leading to places of strategic significance around the world, such as CERN and the Pentagon.

Realising that these portals were projections, Nardole theorised that the places they led to were not in fact real places. To test this theory he went beyond the light of the projection, and disappeared out of the Shadow World. (TV: Extremis)