“C” was the head of MI6. He had previously worked with”O”, but sacked him for some incident.
After operatives both for MI6 and all other spy organizations on Earth were attacked by the Kasaavin, he sent operatives to retrieve the Thirteenth Doctor, Ryan Sinclair, Yasmin Khan and Graham O’Brien. He briefed the group on the attacks and asked them to investigate, believing alien technology was involved. Whilst presenting them with spy-gadgets, “C” was shot dead by The Master. (Spyfall)