How did Avatar construct such realistic characters?

Few cartoons boast the unassailable reputation of Nickelodeon's Avatar: The Last Airbender. Now almost 20 years old, Avatar remains among the most popular animated shows in the West. You can attribute Avatar's success to a myriad of factors, including the Western anime style and the fluidity of combat. However, most Avatar fans will cite how real the characters and their arcs feel in a series designed for children. 

As the Netflix live-action adaptation of Avatar: The Last Airbender inches closer to airing, let's hope the executives behind the series have a grasp of what made the original so popular. At the very least, it can't be worse than the last live-action adaptation. The Royal Ocean Film Society explores how Avatar constructed such realistic characters in the video linked above.