Author's Note: I have been unable to find a specific description of God C's appearance so I have left the description simple, based on pictures I have seen.
God C, as its known in the Schellhas system, is a bit wrongly named. Known also as k'uhul (or ch'ulel), this "god" is currently understood to be some kind of personification of sacredness and not necessarily a god.
God C's appearance is that of a man. Sometimes the head of God C is bearded. One theory exists that God C's appearance is unlike anything that can be found in the natural world, and so is not an anthropomorphic relation to the natural world.
The term k'uhul is understood to mean both divinity and a life force the ancient Maya considered sacred. These two things both existed in the world and were brought into the world via magic rituals.
History of Redefining God C
When Schellhas first made his classification of the gods found in the Maya codices, he thought that the god he termed God C was a simian (monkey) deity. Later a theory existed that stated God C was a god of the north. This theory was replaced by the current understanding.
God C may also have had functions archaeologists have yet to find out. In the Madrid Codex, ancient Maya artists depict God C in various ways that could possibly mean that the god may have a connection to astronomy. Various ways he is drawn in the codex include being placed in a skyband, on a skyband throne or walking on a road with a merchant's bag.
Another possible indicator of God C being related to astronomy is an image in the Madrid Codex that shows Chac (God B) coming out of God C's mouth. This could be an artistic representation of a conjunction.
Missouri State University: MAYA GODS AND GODDESSES
"Archaeology of Ancient Mexico and Central America: An Encyclopedia"; Susan Evans; 2000
"The Ancient Maya: New Perspectives"; Heather Irene McKillop; 2004
"Handbook to Life in the Ancient Maya World "; Lynn V. Foster; 2005
"Star Gods of the Ancient Maya: Astronomy in Art, Folklore, and Calendars"; Susan Milbrath; 2000