Caesar was born Edward Sallow. Caesar's parents were killed at a young age and he was taken in by the Followers of the Apocalypse. The Followers educated Caesar and accepted him as a full member. Caesar would later travel east and study Tribal dialects with Calhoun, another follower. While studying the tribes they met Joshua Graham. While studying the tribes the three men were taken prisoner by the Blackfoot tribe. During this time the Blackfoots were at war with several other tribes. They were losing badly due to inexperience with combat; Caesar saw this as an opportunity and chose to educate the tribe on the maintenance of guns, unit tactics, and the manufacturing of explosives. The tribe was so impressed by his knowledge and tactics that they elected him as their new leader. One by one the tribe conquered all the remaining tribes under Caesar's rule. As the tribes were conquered they were either integrated into Caesar's tribe or enslaved by the tribe. These tribes would later form The Legion.
The Legion would continue to conquer unopposed until their first encounter with the NCR at the First Battle of the Hoover Dam. Caesar's Legion was defeated at the Dam and has since been posed at the far end of the dam waiting for an opportunity to seize the dam.
Role in Fallout: New Vegas
After making enough progress on the main quest The Courier will be approached by a messenger sent by Caesar, inviting him to The Fort. It is at the Fort that Caesar asks The Courier to help him seize power on the Strip. Caesar believes that the NCR is doomed to fail due to their disjointed organization. He firmly believes that wiping them out is just hastening the inevitable.
Should the player decide to side and work with Caesar, they'll notice over time that he's experiencing severe pains in his head. With a high enough medicine skill, the player can diagnose him with a tumor. Alternatively, they can convince him with a high enough speech skill that there is a problem with his head and that he needs surgery. Either way, the player will get a quest where they must fix his Auto-Doc using parts from another one (located in one of the Vaults) and buy surgery tools to save his life (for several thousand caps). Alternatively, once the Auto-Doc is fixed, the player can set it to "Autopsy Mode", which kills Caesar.