You need to unlock Terrako first I think, and you'll be able to get a mission called "Versus Calamity Ganon". You get him from that. Just keep cracking away at postgame and he'll show up eventually.
So wait, is Calamity Ganon part of the DLC or no? Because I remember the DLC saying that Robbie and Purah and one other person were new characters. Terrako is base game and one of my friends who doesn't have the DLC says she has Ganon.