From Halopedia, the Halo wiki

'D' was likely the first initial or other designation of an operative working for the Office of Naval Intelligence. Over a period of eight years, he and his subordinate, 'S', had been tracking the actions and whereabouts of the Banished and their leader, Atriox, until their trail ran cold in March of 2558. 'D' was eventually fired or removed from his position by this time, possibly by ONI’s Section Zero, which had been secretly surveilling the two operatives’ communiques.[1]

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