Count Iblis was an exiled alien who was rescued by the Battlestar Galactica and stirred trouble among its crew until he was ejected from the ship: see this link. His name was inspired by the demon Iblis in Islamic mythology.
When an expedition from the Galactica lands on an unexplored planet, they find Iblis, standing amidst the wreckage of a huge starship which had crashed on the planet some time before. (In scenes deleted from the final episode, some of the dead crewmen of this ship are seen - clearly resembling the Devil, with cloven hoofs and pointed ears.) Iblis is taken back to the Galactica, where he quickly grows to be very popular among the crew. He is charismatic and likeable, and enthralls the Council of Twelve so much that they intend to elevate him to the presidency. Iblis even arranges for the capture of Count Baltar which only endears him more to the crew. Iblis also says that he has been to Earth but will not give the coordinates for where that planet can be found.
Later, Captain Apollo and several other Galactica pilots are captured by a huge 'Ship of Lights' which appears to be floating in space. The crew of this Ship are angelic beings who tell Apollo and crew that Iblis was originally one of them, but was cast out when he tried to lead a rebellion and get others to follow him. Apollo takes this information back to the fleet, where Iblis is already attempting to stir up a similar situation. Iblis becomes enraged and tries to attack, but Apollo fires on Iblis, who departs after realizing that he has no power other than that which others freely give him (similar to Mormon concepts of the devil).
The beings from the Ship of Lights subsequently reward the Galactica crew by providing them with the ___location of Earth.
It is implied in this episode that Iblis visited the ancient Cylons thousands of yahrens ago, and that the Imperious Leader (or its android duplicate, specifically) was constructed to resemble Iblis - Count Baltar even observes that Iblis' voice is also that of the Imperious Leader. (No surprise there, since both characters' voices were provided by actor Patrick Macnee.)
Iblis has not yet appeared in the recent remake of the show, although rumors continue to circulate that Iblis will somehow make an appearance (many fans believe that Iblis is the Cylon 'god' frequently mentioned by Number Six).