The Monitor

The Monitor was a Logopolitan and served as the interface between visitors and the other Logopolitans (whether this made him the leader or not is unknown).

The Monitor was a good friend of the Doctor and kept Logopolis' secret from him at all times. When the Master arrived on Logopolis in 1981 and silenced Logopolis, the secret was revealed: the Logopolitans had been staving off the end of the Universe, and now that they were dead, entropy would build up and destroy the universe.

In a final attempt to gather all information on programming the Charged Vacuum Emboitments, which had prevented the end of the Universe, the Monitor was disintegrated by the entropy. (DW: Logopolis)