Cloud: Set up by using the Defend command, then continue fighting as normal. The next time Cloud is hit by physical damage he will counter it with either a single physical attack, a Deathblow, 2x-Cut, or 4x-Cut. This works in addition to any other counter materia.
Barret: When hit by a physical attack, if in the front row he gains Strength (up to +40). If in the back row, he regains 1/16 of his HP.
Tifa: Once per battle, she will survive a KO with 1 HP (or ignore a insta-KO attack) and retains any queued actions, buffs, Limit Gauge, etc. However she also retains negative statuses so Poison or Dual-Drain will undermine this innate.
Aeris: Activate by Defending, Aeris will absorb most elemental damage and HP while she defends.
Red XIII: Over time will gain strength and magic up to a max of +35 for both; is useful in longer battles.
Yuffie: If attacked physically while in the front row, she will counter with Mug. One enemy in the battle will have their given Gil doubled (or set to give a small amount if Hard Mode is enabled where Gil gain is 0).
Cait Sith: Regains MP slowly over time; when Defending, the entire party will regain MP at a faster rate.
Vincent: Starts battle with Wall; will intermittently recover HP equal to his current MP.
Cid: After some time has passed in battle, a message saying 'Cid Mix is Ready'. Using the Defend command activates Mix with the cascading series of effects:
If Cid's HP is below 50%, he will use H-Potion and recover 25% of the party's HP.
If Cid's MP is below 50%, he will use I-Ether and recover 25% of the party's MP.
If Cid does not have Barrier or MBarrier he will use G-Curtain and gain Wall.
If none of the above conditions are met, he will use H-Mine which is a physical attack with 24 base power (standard physical base is 16).