Emil Zatopek was Professor Martinique's assistant. He was also a latent psychic and an agent of the Brotherhood.
He was part of Professor Martinique's expedition to Iphigenia. Exposure to the Nexus there caused an alternate Seventh Doctor to merge with him. They went to Cassandra, which they learned was a damaged TARDIS, and activated its self-destruct so it would not fall into the wrong hands. The Brotherhood eventually abandoned him when he was no longer of use. The possibility of the alternate Doctor diminished over time, eventually reaching zero and killing Zatopek after two weeks. (PROSE: So Vile a Sin)