Two's name was based on rank; he was second only to One. His communicator had the number 2 written on it, an indication of this rank. Two scouted Refusis II with the First Doctor, Dodo Chaplet, and Yendom ahead of the main Monoid landing party. A Refusian used force to prevent him breaking objects in a rage.
Two was forced to kill Yendom after Yendom discovered the Monoids weren't going to take the Guardians down to Refusis.
Two reported back to the Ark and was about to tell them about the Refusians when a Refusian killed him by having his landing craft explode. When the main party landed, the other Monoids found the wreckage of the landing craft. (TV: "The Return")