Yakone

Yakone is a villain in The Legend of Korra, who was mentioned by Councilman Tarrlok in the episode "The Voice in the Night".

Tarrlok mentioned him in order to persuade other council members in the United Republic Council to allow his task force to face Amon head-on, just as Aang had done to Yakone.

During the show, which is set after the decease of Avatar Aang, Yakone (or what appears to be him) and Aang appear through flashbacks during a few of Korra's defeats. What purpose he serves in the series is yet to be known, but it is implied he was defeated by Avatar Aang when he threatened Republic City in 128 ASC.