Dr. Gaius Baltar was a man haunted by a secret: he was the one who caused the Fall of the Twelve Colonies.
It all began when he started an affair with a blonde woman in a red dress. He thought this mysterious woman was working for a rival defense contractor, so he gave her access to the Colonial defense mainframe, in exchange for her assistance in creating a navigation program for the Colonial Fleet. That was before he found out that this woman was an advanced Cylon infiltrator known as Numbers Six, designed to resemble their human counterparts—and he just helped the Cylons launch their campaign of genocide against the Twelve Colonies.
Baltar survived the nuclear assault and ended up becoming a special advisor to the newly-inaugurated president Laura Roslin. However, Baltar would be constantly haunted not only by his own guilt over his role in the Fall of the Colonies but also by a ghostly hallucination of Number Six and would be driven to conceal all evidence of his involvement in this catastrophe.