Note : All of the following characters' Reaction, Support and Movement Skills are those of the Squire, so only the Action Abilities will be listed here.

Here they are in the order you encounter them :

Boco (Chocobo) You'll meet him in Araguay Woods, where he is cornered by Goblins. He is useful since you might get Black or Red Chocobos from the eggs he produces. Other than that, once you get better Chocobos, you can kick him out if you wish.
Action Skills : Check the Monsters section to see his abilities.

Mustadio (Engineer) He is being pursued by the Bart Company for unkown reasons. He is able to use a Gun and his special skills are more helpful than you think. Once he gets ahold of a Magic Gun, he'll be very powerful indeed. He joins you as a playable character after the battle in Goug Machine City.
Action Skills : Snipe

Arm Aim : If succesful, the target won't be able to act for a few rounds. Best used with a gun.

Leg Aim : If succesful, the target won't be able to move, leaving him vulnerable to spells. Best used with a gun.

Seal Evil : Works wonders on Undead monsters, not only does it perify them, but it also keeps them from reviving. Best used with a gun.

Agrias (Holy Knight) She will be with you in the beginning of chapter 2, but only joins you as a playable character after the battle in Bariaus Valley. She is very strong, don't refuse her help.
Action Skills : Holy Sword

Stasis Sword : Holy elemental attack, it damages plus randomly stops the target(s).

Split Punch : Holy elemental attack, it damages plus randomly gives Death Sentence to the target.

Crush Punch : Holy elemental attack, it damages plus randomly kills the target.

Lightning Stab : Holy elemental attack, it damages plus randomly silences the target. Very helpful against magic users.

Holy Explosion : Holy elemental attack, it damages plus randomly confuses the target.
Rafa (Heaven Knight) She'll join you when you go to Riovanes Castle. Her low brave makes her the best Item Finder. Aside from that, she has no use to the party. She is Malak's sister by the way.

Action Skills : Truth

Heaven Thunder : Lightning elemental attack, which hits random panels in attack area.

Asura : Fire elemental attack, which hits random panels in attack area.

Diamond Sword : Wind elemental damage, which hits random panels in attack area.

Hydragon Pit : Water elemental damage, which hits random panels in attack area.

Space Storage : Damages plus causes status ailments, which hits random panels in attack area.

Sky Demon : Earth elemental damage, which hits random panels in attack area.
Malak (Hell Knight) Malak joins you after the Riovanes Castle events, his brave is high but his faith is very low, unlike Rafa, he has no use to the party. Dismiss him right away. He is Rafa's brother by the way.

Action Skills : Un-Truth

They're the same as Rafa's skills, no need to rewrite them.

Orlandu (Holy Swordsman) Orlandu is THE most powerful member you'll get, no enemies (except Hydra and Tiamat) stand a chance against him. He has the same sword skills as Agrias, Meliadoul and Gafgarion. Only much more stronger.

Action Skills : All SwordSkill

Holy Knight's Skills (Agrias) Look up Agrias's abilities, they're just stronger with Orlandu.

Divine Knight's Skills (Meliadoul) Look up Meliadoul's abilities, stronger with Orlandu.

Dark Knight's Skills (Gafgarion)

Night Sword : Takes away enemy's HP and restores own HP. Do not use against Undead.

Dark Sword : Takes away enemy's MP and restores own MP. Do use this against magic users.
Meliadoul (Divine Knight) At first, she beleives Ramza killed Izlude at Riovanes and wants to avenge her brother's death. When she see Elmdor becoming Zalera, she has doubts of her father's intentions. She is the daughter of Vormav, the leader of the Shrine Knights. The good reasons to take her along are her sword and high faith.
Action Skills : Mighty Sword

Note : Meliadoul's Sword Skills can only affect humans, no monsters.

Shellbust Stab : Damages plus destroys Armor.

Blastar Punch : Damages plus destroys Helmet.

Hellcry Punch : Damages plus destroys Weapon.

Icewolf Bite : Damages plus destroys Accesory.
Beowulf (Temple Knight) Beowulf is an optional character, if you choose to the Cloud SubQuest, he'll join you to go in Goland Coal City. He is much like an oracle, but since the spells don't need to charge, it helps alot. He's worth getting.

Action Skills : Magic Sword

Blind : Causes Darkness status. Uses 6 mp.

Aspel : Drains enemy mp. Uses 2 mp.

Drain : Drains enemy hp. Very useful against Altima's last form. Do not use on Undead. Uses 12 mp.

Faith : Causes Faith status. Uses 10 mp.

Innocent : Causes Innocent status. Uses 10 mp.

Zombie : Causes Undead status. Uses 14 mp.

Silence : Causes Silence status. Uses 16 mp.

Beserk : Causes Beserk status. Uses 16 mp.

Chicken : Causes an enemy to lose up to 50 Brave points. Uses 12 mp.

Confuse : Causes Confusion status. Uses 14 mp.

Despair : Removes positive status from enemies. Uses 20 mp.

Don't Act : Causes Don't Act status. Uses 14 mp.

Sleep : Causes Sleep status. Uses 20 mp.

Break : Petrifies an enemy. Uses 24 mp.

Shock! : The less HP Beowulf has, the more damage Shock ! does. Uses 20 mp.

Reis (Holy Dragon) You'll get her when you suceed in saving her from Sinogue in the Goland Mines. She only has 3 attacks, but they're powerful.

Action Skills : Breath

Ice Bracelet : Does Ice damage to one enemy.

Fire Bracelet : Does Fire damage to one enemy.

Thunder Bracelet : Does Lightning damage to one enemy.

Worker 8 (Steel Giant) You'll gain Worker 8 after you've saved Reis from Sinogue. He only has 4 attacks and everytime he uses them, he loses a bit of hp. Still, his attacks are very strong.

Action Skills : Work

Destroy : Close range physical attack.

Compress : Kills one enemy at close range.

Dispose : Long distance physical attack.

Crush : Another close range physical attack.

Reis (Dragoner) She'll revert to her human form after you beat Worker 7 in Nelveska Temple. She still has her breath attacks and gains some new attack, but they only affect dragons or hydras. She has many inherant skills like the Ninja's Two Swords, the Mediator's Train and the Squire's Monster Skill. The only bad thing about her is that she has little movement, give her Germinas Boots to make up for it.

Action Skills : Dragon

Ice Bracelet : Ice damage to one enemy.

Fire Bracelet : Fire damage to one enemy.

Thunder Bracelet : Lightning damage to one enemy.

Holy Bracelet : Holy damage in random panel within attack area.

The following skills only affect dragons or hydras, they won't work on any other enemies.

Dragon Tame : Similar to the Mediator's Invitation, but a 100 % succes rate on dragons.

Dragon Care : Gives some hp to ally dragon or hydra.

Dragon Power Up : Dragon ally gains some strenght.

Dragon Level Up : Dragon ally gets its CT to 100 so it can attack right away.
Cloud (Soldier) Yep, it's Cloud of FF7. Look up the How to get Cloud section. His usefulness will depend on how much time you spend building him up. At level 70, he's nearly as useful as Orlandu. Have fun with him. ;)

Action Skills : Limit

Braver : Simply damages the enemy.

Cross-Slash : Magic attack to one or multiple enemies.

Blade Beam : Similar to Beowulf's Shock, the less HP Cloud has, the more damage it does.

Climhazzard : The less HP the target has, the more punishment it'll take.

Meteorain : Meteors come crashing on the enemies, doing lots of damage.

Finish Touch : Either petrifies or stops targets, thus being Cloud's most useful Limit.

Omnislash : Another very strong Limit, hits one or multiple enemies.

Cherry Blossom : No, this ain't those chocolate candies with a cherry ^_^, it's Cloud's ultimate Limit, you silly things. :-P Does Lightning/Fire/Ice damage to one or multiple enemies. Cast Don't Move on your target first.
Byblos (Byblos) You'll get him after you've beaten Elidibs, and only if he was still alive. He's pretty decent since he can even heal Undead allies and poaches any monster it kills.

Action Skills : Byblos

Energy : A stronger version of Ramza's Wish, it heals up to 250 + hp to one ally, but he lose half the amount of hp that cured you.

Parasite : Causes multiple status ailments on one enemy.

Shock ! : Same thing as Beowulf, less hp, more damage.

Difference : The more mp the enemy has, the more damage it does. Useful with magic users.