Ah screw i'll do a few to make a point...
Locke: Celes love interest for one. I mean, i could go on... but that's good right? I mean some consider him the main character!
Strago: One of the few who know the secrets of the Espers... a good paralel to Terra. Since he has magical powers naturally too... but can feel.
Cyan: A living reminder of Kefka's Every time he shows up he reminds you how heartless Kefka is.
Shadow: Already explained
Sabin: Edgars brother. He contrasts Edgar... without Sabin, Edgar's character and personality would be lost as simply just an egotystical ladies man. Without Sabin Edgar isn't the same character.
See the theme here? The support characters support the story and it's characters.
Edgar isn't the same without Sabin.
If you get rid of Red XIII who does that effect?
Yuffie, Caith Sith? If you completely removed them from the game.... who do they change?
They are supporting characters who support nothing.