VI features the largest cast of playable characters in an FF. They are all really good and most of them are broken. I recommend switching out and trying new party combinations. Don't neglect too many characters as the game has dungeons that require you to use most of the cast.

Relics are another system in the game. They are basically accessaries that add additional bonuses but many can dramatically change a characters ply style or grant new abilities (Example: Dragoon Boots change the Fight command to Jump, and the Genji Gloves grant the ability to Dual Wield). Their pretty limited in the beginning but after awhile you can really start customizing your party.