When you play in the WOR, you start with Celes, You have to meet up with Edgar to get Figaro running so that you can get to Kohlington, and you have to meet up with Setzer so that you can get the airship and jump to Kefka's Tower. So you have to have those three people at a minimum to beat the game.

What Outsider is saying is that in playing through the game, you will have all of the characters, minus Gogo and Umaro, in your party at one point or another. And that in the WOR, all characters are optional other than Celes, Edgar and Setzer, as you have to have them to attack Kefka's Tower.