From the intro, you can see the silhouette of the Earthbender where there’s an unmistakable beard and man-bun. That’s not a ton to go off of during the beginning of the series, but on Episode 46, “The Avatar and the Fire Lord,” we’re introduced to an old Earthbending master by the name of Sud. He taught Roku, who was another Avatar before Aang. Sud showed Roku the ways of Earthbending until the student was finally able to overcome the teacher.
That flashback sequence is so far the only time Sud has been seen in the “Last Airbender” franchise. From what we see and hear from him, Sud is very much like other Earthbenders before and after him. He’s described as “uncompromising, stubborn, and blunt,” which are all attributes possessed by Aang’s Earthbending teacher, Toph, too.
Sud and Toph are more closely linked than you likely thought initially. Translated, “Sud” means “South” in Italian. Conversely, Toph’s last name is Beifong, which is close to the way “North” is pronounced in Mandarin. This could suggest that it’s not just the Avatar who’s reincarnated. Perhaps over the course of a century, Sud reincarnated into Toph, or at the very least, the two are connected in some way.