Marplon was a male inhabitant of Beta III in the 23rd century. As an an official, his duties included the absorption of new members. He was also one member of the underground that opposed the will of Landru.
In 2267, when the USS Enterprise visited the planet and became trapped, he provided vital assistance to the landing party; he prevented Kirk and Spock from being absorbed, and he returned their weapons and communicators to them. After Reger collapsed emotionally, Marplon guided the landing party to the Hall of Audiences, so they could confront Landru.
As a trusted official, Marplon evidently had access to more information than was available to most members of the Body; he knew which of the devices removed from the landing party were weapons, and which were communications devices. He also knew how to operate the absorption chamber, and how to substitute a harmless light show for the absorption process. Nevertheless, he held Landru in nearly the same reverence as a member of the Body would -- hesitating at the Hall of Audiences, and displaying shock when Kirk and Spock revealed that Landru was actually a sophisticated computer. (TOS: "The Return of the Archons")