Dekker

Dekker was the Head of MI5's Technology division, who also was in charge of alien monitoring; Dekker had been constantly monitoring the airwaves for the possible arrival of the 456 since the 1960s and knew far more about the aliens than anyone else.

Biography
On Day One Dekker discovered that once again the 456 had contacted the world via the airwave on an old computer from 1965. Almost immediately he contacted civil servant John Frobisher, telling him about the 456's return. Dekker was in the Ambassadorial Suite with the 456 several times over the five days, fascinated by the alien and what it really was.

On Day Four when Captain Jack Harkness angered the 456 and it released poisonous gases through the building, Dekker escaped to his office and put on a HAZMAT suit which allowed him to breath non contaminated air, making him the only person in the building to survive – Captain Jack died, temporarily. By the time of Day Five; Dekker was the only MI5 member left in Thames House, with most of MI5 having been killed the night before, Dekker was likely placed in charge of MI5 temporarily.

On Day Five, Dekker greeted Colonel Oduya of UNIT who took over the negotiations with the 456. Later when a small mutiny led by Agent Johnson took place, Captain Jack was released from prison and Dekker was knocked out and taken to Ashdown House where he was shot in the leg by Johnson after he complained.

Dekker took great malice in informing Harkness that the child at the centre of the wave to destroy the 456, which would have to be Harkness' own grandson, Steven Carter would be killed in the process (going as far as laughing when he said "The kid's gonna fry"). Steven did die during the process and Dekker was shown to be saddened. (TW: Children of Earth)