The Legend of Korra brought back a lot of beloved characters from the original Avatar: The Last Airbender series and gave them prominent roles in the future. Katara is present from the very beginning, while Zuko and Toph rise to prominence in the later books. Toph in particular becomes a regular character in the final season of the show. Aang is, of course, not still alive, as he must have died in order for another Avatar to have been born. Even so, he still appears as an Avatar spirit present within Korra.