From his speech Jeremiah is obviously a native of Georgia. From what he speaks of, he is obviously an educated man. It's too bad that Jeremiah is usually a no-good sonofabitch. He's dueled men who've accused him of cheating at poker, taken jobs of dubious moral value and even operated a brothel for a spell until he was muscled out. Currently in the Weird West, Jeremiah makes his living as a gambler when times are good, and as a hired thug in the rare instances when the cards turn against him for a spell. His education has also proven quite useful in the uncouth West, and people will turn to him for advice when he's not drunk.
Jeremiah was born to the wealthy Rathbourne family of Savannah, Georgia. Despite the war, Jeremiah was sent to Harvard for his education, though he was expelled for gambling and philandering in his senior year. Still living off of his family's wealth, he moved to New York and frequented the seedier parts of that city. Soon he began to study Hoyle's Book of Games.
Jeremiah had a natural talent for hexes, which caught the attention of certain unsavory elements in New York, especially Boss Tweed and his gang-enforced political machine. Jeremiah killed two of Boss Tweed's men in self defense after refusing a job for Tweed and fled West to avoid an influenced murder trial, taking the name MacGregor in place of Rathbourne. There is still a sizable reward offered by New York state for the "Murderer and Foul Warlock: Jeremiah Rathbourne."
He will move from time to time as a new Wanted poster from New York arrives, though from his behavior few would recognize Jeremiah as the Harvard-educated scion of one of Savannah's leading families that the posters describe. Still, occasionally a hex goes bad and Jeremiah gets noticed, and so he moves on with his trusty horse, Peaches. His family has also hired the Pinkertons to find their missing heir.