Okay. First time I heard the term deuteragonist.

I would disagree with Kefka being the protagonist as I see him squarely as the antagonist.

I see Terra and Celes as joint protagonists. The spotlight is on both of them as the leading mages and in the World of Ruin, Celes is the character you begin with, and those scenes she has are so powerful that I give her equal billing with Terra. The weird thing is that Terra becomes optional here, which stops her from being the sole protagonist.

I see Zidane as a full protagonist as he is as integral to the plot as Cloud or Squall are in their stories.

Then, you have Tidus as the lead in FF10 and Yuna goes from whatever that word Kanno posted that I can't be bothered to look and check, to the protagonist of 10-2.