Sud was a citizen of the Earth Kingdom when Roku was a young man. He was a skilled and powerful earthbending master given the honor of teaching the Avatar earthbending, the last element Roku needed to master in order to become a Fully Realized Avatar. Roku described him as being "uncompromising, stubborn, and blunt", which are the very qualities that define earthbenders.
Once his training was completed, Roku beat his teacher in a race up a mountain by surfing on rocks. Sud seemingly pulled ahead at first; however, when he reached the top of the hill, Roku was already sitting there brewing a pot of tea and offered him a cup. Sud laughed and they sat down to enjoy the hot beverage. Having mastered all four elements, Roku journeyed back to the Fire Nation in order to master the Avatar State. Roku and Sud remained very good friends afterward.
Of the four shadowed benders featured in the opening sequence, Sud is the latest, and currently the last, to have been identified, after Pakku and Azula.
Sud's character design was confirmed by Mike and Bryan to be Toph's original design.
When translated from French, "Sud" means "south". It is interesting to note that to be "south" is to be resistant to change or to be stubborn with new ideology, which are two main principles involved in earthbending, and both main parts of Sud's personality. This also made him a counterpart to Toph, as her last name Beifong means "north".