After reaching the World of Ruin, you will first play as Celes alone. You will, from there, begin reassembling your vast party. However, if you have the patience, when it's still just Celes in your party, of working her up to level 99, then all of your characters will also be level 99 when you're reassembling your party. It's worth the effort if you want to have a ridiculously powerful party in the World of Ruin.