Terra, Locke, Celes, Edgar, Sabin, Cyan, Setzer, shadow, relm, strago, mog, gogo, gau, umaro.
 
these are the characters you can have during your adventure.
 
At one time or another you had to have most of these characters in your party except umaro and gogo.
The only secret character at this point in the adventure one can have through means other then them just joining is gogo whom can be found in the belly of a zone eater on the solitary island in the north east of the map, Umaro is found later on in narshe mines but only if mog is in your party.  locke and celes being the only ones in your party at the opera scene is a natural part of the game...if you have literally only celes and locke in your party then i dont know what you've done wrong....though its been a long time since i played ff6...

Having Celes and Locke in the opera house is normal. After the opera you get everyone back I believe.

" Terra, Locke, Celes, Edgar, Sabin, Cyan, Setzer, shadow, relm, strago, mog, gogo, gau, umaro.  these are the characters you can have during your adventure. At one time or another you had to have most of these characters in your party except umaro and gogo. The only secret character at this point in the adventure one can have through means other then them just joining is gogo whom can be found in the belly of a zone eater on the solitary island in the north east of the map, Umaro is found later on in narshe mines but only if mog is in your party.  locke and celes being the only ones in your party at the opera scene is a natural part of the game...if you have literally only celes and locke in your party then i dont know what you've done wrong....though its been a long time since i played ff6... "

The characters that you'll have available to you will fluctuate for most of the first half of the game.

" @WilliamRLBaker said:

" Terra, Locke, Celes, Edgar, Sabin, Cyan, Setzer, shadow, relm, strago, mog, gogo, gau, umaro.  these are the characters you can have during your adventure. At one time or another you had to have most of these characters in your party except umaro and gogo. The only secret character at this point in the adventure one can have through means other then them just joining is gogo whom can be found in the belly of a zone eater on the solitary island in the north east of the map, Umaro is found later on in narshe mines but only if mog is in your party.  locke and celes being the only ones in your party at the opera scene is a natural part of the game...if you have literally only celes and locke in your party then i dont know what you've done wrong....though its been a long time since i played ff6... "

I did ask for no spoilers, didn't I? "

I think you can still go back to Zozo and rejigger your party at this point.