The metrics that define a character are not necessarily clean-cut in Final Fantasy VI, and it took the advent of the modding community before the ins and outs of all of them were finally clear. Good thing they did that, because now you can read about them.
Attack is only raised by the weapons you equip, and is a factor in determining Attack damage (not Desperation Attacks, though). It's also used with Umaro's physicals.
Defense is only raised by equipment and Relics and is a factor in determining physical damage taken by your characters.
Evasion handles how often you block/evade physical attacks in a situation where you can block physical attacks. Evasion above 128% is useless, as at 128% Evasion the character will block every blockable physical attack that is used against him or her.
Though the percentage can be confusing, treat the Evasion stat as such, on a scale of 128 instead of 100. 64% Evasion means that exactly half of the physical attacks are evaded, for instance.
Magic is a factor in determining the damage you do with magical attacks. Magic of all stats has the most influence in damage output, which is why it's usually the best stat to raise on characters. Primary magical attacks for your characters include:
Magic Defense is only raised by equipment and Relics and is a factor in determining the amount of damage taken from magical attacks.
Magic Evasion handles how often you block/evade magical attacks in a situation where you can block magical attacks. Magic Evasion above 128% is useless, as at 128% Magic Evasion the character will block every blockable magical attack that is used against him or her.
Though the percentage can be confusing, treat this stat as such, on a scale of 128 instead of 100. 64% Magic Evasion means that exactly half of the magical attacks are evaded, for instance.
Speed is a determining factor in your ATB gauge. Characters with a lot of Speed have an ATB bar that fills up faster than character with less Speed. However, Speed is a fairly worthless stat to raise, as the change is rather insignificant and you'll need to raise the Speed stat a lot if you ever want to double-turn another of your characters.
Stamina is a minor stat that determines HP gained by Regen, as well as HP lost by Poison and Sap. The more Stamina a character has, the more HP will be gained/lost. A second influence of Stamina is the amount of HP a Tintinnabulum will heal per step, although take heed: Stamina given to you by equipment will not be taken into account by the Tintinnabulum. The final and most important use of Stamina is evading one-hit KO attacks. More Stamina means more evasion of them. The spells that check for Stamina are the following: Break, Death, Gravity, Graviga, Banish, Tornado, Catoblepas, Odin, Raiden, Antlion, Snare, Banisher and Gravity Bomb. Stamina is a complete waste of your time. While 128 Stamina will ensure none of the above attacks will ever make a hit, it's not worth it.
Strength is a factor in determining the amount of physical damage your character will deliver. Obviously, more Strength means more damage. Strength, however, is only a small factor in the formula, so boosting it with Espers and Relics (Hyper Wrist) will make little impact. Primary physical attacks for your characters include:
Use these hunks of info to buff your characters as desired. Go on, then.