Miller is the calculating spymaster from an unnamed power who masterminds the plot to hijack the Syldavian rocket programme in Destination Moon and Explorers on the Moon. He was probably the man who offered to help Frank Wolff out of his gambling debts in exchange for information on the rocket programme: Miller is shown in one scene checking a list of personnel at the Centre and presumably finds Wolff's name among them.
Miller is first seen on the plane to Syldavia in Destination Moon. He was seated in the row ahead of Tintin and Captain Haddock and was astonished to hear the Captain mention the name "Calculus", showing that he was already planning to take over the moon programme that Calculus was working on. He discreetly followed Tintin and Haddock through Klow airport but pulled back when he realised that they were being escorted by agents from ZEPO, the local secret police.
With an associate known as the Baron, he then set about parachuting agents into the area of the Centre and obtaining the plans for the experimental unmanned rocket X-FLR6 from Frank Wolff. When X-FLR6 was launched, Miller's technicians were able to intercept it and divert the rocket to their own territory. However, Tintin and Calculus had expected this and destroyed the rocket before it could land.
Miller threatened to have Wolff killed, as he suspected him of double-crossing him, but refrained when it was announced that a manned rocket was to go to the moon. Miller arranged for Colonel Jorgen, an old enemy of Tintin's, to be smuggled aboard. He himself stayed up-to-date with events by listening into radio transmissions between Earth and the rocket as it travelled to the Moon and back. Ultimately though, the attempt to hijack the rocket failed, both Jorgen and Wolff perishing in the process. Miller is last seen cursing the rocket's crew and his agents' bungling, wishing that they would all perish in the last stage of the return journey.
Like any good spymaster, Miller designated various codenames to his targets and operations: the Sprodj Atomic Research Centre was referred to as the "Main Workshop"; Calculus and Haddock were "Mammoth" and "Whale"; and the operation to hijack the manned rocket to the moon was called "Ulysses" (after the Mythical Greek hero who also goes on an epic journey and is himself a master of intrigue and deception).