gogo can do everything(except increase his stats) Mog has the moogle charm, gau is possibly the best in the game if used right Strago is a blue mage which is always good and character wise cyan is one of the deepest most brilliant ever created.

I do agree about umaro and relm but part of the reason VI is so great is the different ways to play through it using different characters and the fact that besides terra being the "main" character, almost everyone is equally important in the grand scheme of things.

Of course, your just answering the question but VI is the hardest for me to even identify a second string... Hmm... yeah relm, umaro and setzer.