Roku's waterbending master was a citizen of the Northern Water Tribe when Roku was a young man. He was a very skilled and powerful waterbender, as he demonstrated great proficiency in the art, eventually earning the title of waterbending master. Later in life, he was given the honor of teaching the Avatar.
At first, Roku experienced difficulties with learning waterbending, as it was the element opposite his native element, fire. However, after several years, Roku mastered the element, even being able to defeat his instructor in a duel.