World of Ruin is one of the greatest things to happen in gaming history.
It's a year after you lost to the comedy relief secondary antagonist and you control a secondary character who is trying to kill herself by jumping off a cliff.
Somehow, SOMEHOW, you have to claw your way off that beach and rebuild your party against all the odds.
One main character is at the bottom of one of the hardest dungeons in the game with mechanics that almost guarentee you are getting him last. The other main character you find early but lost the will to fight and just doesn't want to join you, oh and winning may just cause her death.
Everyone has gone through some shit and changed... needing rescue or character development to join you. Except fucking Sabin, who is wandering the world just being Sabin.
At the floating island in replaying 6 pixel remaster version - This is the first FF game where a "a message(s)" of sorts can be better conveyed in a game - while most point to 7 for that, and there are obviously plot and themes in all before it- 6 really has the characters and story to give some real beef to everything.
I will give tree-bro-ex death a 3rd place award for actually doing big bad blow up the world stuff, with 2nd place being FF9 - but 1st place always is Kefka and absolutely destroying the world and throwing many of your characters into a living hell and depression.
Terra is obviously depressed and has lost her will to fight - but she also found love, something she thought she saw and wanted in Leo - but she found it in motherhood and fostering the village of children... it was that that she held on to and used as a veil for not fighting, until she is forced to realize she must fight in order to love and protect her children (and, I suppose the world)
FF6 is better moral messages imo, just not as apparent or clearly stated as what many claim praise on FF7 "save the earf bro, nuklar power is bhad, big company bros are the evil." hippy messaging.
I think is also better done by the larger cast of FF6, something no FF game has done since because...reasons.
But Celes is not really secondary, nor is Terra primary - they are both, Earth (Terra) and Heavens (Celestial) both are leads - Terra is just presented first and most naturally attune to that. Again I will rant about how there has always been "female" leads in video games - people just didnt make it a deal, and if you point back at them they hand-wave it off and still demang GURLPOWAR.
FF4- Rydia has nearly as much development as Cecil and Kain. 3 Main cast are female.
FF5 - Bartz really is just there, sure there is some development mid game of him being the son of one of the dudes, but really the princess and lost princess are a primary focus and then Gaulf's arc is all to lead to his granddaughter taking over - 3 of 4 characters in your party are female.
FF6 - Terra and Celes are the main character(s) - all the rest of the bro's are really support characters to either or both of them.
FF7 - Tifa used to be big enough to take on all roles, till she got modernized.
FF8 - Sucks