Theopolis, or Theo for short, was an intelligent computer who was part of Earth's Computer Council. He was assigned with the process of assimilating Captain William "Buck" Rogers when he first arrived on Earth after having been in suspended animation.
Theo's appearance was that of an illuminated face in a round container which was shaped like a large disk. He had no means of independent locomotion, and so was often seen carried by Twiki, his assistant. Twiki would typically attach Theo to his chest.
Theo had an important role on Earth as a sort of liaison between the Earth Defense Directorate and its leader Dr. Elias Huer, and the Computer Council, which is the planetary ruling body. Theo was considered one of Earth's leading scientists.
Theo and Twiki were clearly conceived as a similar pairing to the C-3PO and R2-D2 of Star Wars personalities from the successful Star Wars films. Theo would often express exasperation at Twiki who over time developed more and more human mannerisms.
The character of Dr. Theopolis did not return in the second season of the series. However, Twiki did return and the role that Theo did play vis a vis the ambuquad was replaced, at least in part, with the introduction of a new robot character, Crichton.
Eric Server voiced the character for all of the first season, with one exception. In "Awakening", the very first episode of Buck Rogers, the character was voiced by Howard F. Flynn.