Jubal Early, the bounty hunter (played by Richard Brooks)
Jubal is a (rightfully) arrogant bounty hunter who snuck on board Serenity and quickly disabled her crew in a search for the fugitive River Tam. Early had been tracking Serenity since the heist on Ariel and finally catches up to them in the brilliant Objects In Space.
There are any number of parallels between Jubal and River, and we find out on the DVD commentary that many of them were intentionally written in.
Jubal Early is the name of one of Nathan Fillion's ancestors, a renowned southern general(approve sites) of the American Civil War. Nathan mentioned the name and the general's story to Joss Whedon in the course of filming, and Joss used the name for this character.