Beowulf
Beowulf is a Templar recruited as part of a sub-quest started by Mustadio in Goug. He volunteers to help Ramza in Goland against the monster uprising. He joins along with Reis after completing the Goland section of Mustadio's subquest. Beowulf's special ability as a Templar is to cast status ailment spells through his sword, similar to the Mystic class. Beowulf's version of these spells can only hit one target, but he does not have to spend time casting the spells.
Reis
Reis is the fiancé of Beowulf. Rescuing her from the Goland mines is his secret motivation for helping Ramza. Initially, she appears to be a dragon, but it turned back to a human near the end of the quest. Her default class is the Dragonkin. She has the highest base stats on any character while using this class. Her abilities in this class include dragon breath attacks, persuasion techniques and buffing abilities.
Construct 8
Construct 8 is the final secret character recruited as part of Mustadio's sub-quest. He is reactivated in Goug after recruiting Beowulf and Reis. He does not join your party fully until you go to Nelveska Temple, the next part of the sub-quest. Construct 8 is the only non-human secret character. This means he cannot change his job or learn new abilities. His special attacks do large amounts of damage, but also damage him in the process.
Cloud
Cloud, the protagonist from Final Fantasy VII, is the last secret character in the original game. You need to purchase a flower from the flower girl in Zargidas to start the quest to get him. Going to Goug causes Cloud to be transported in Zargidas, where he is encountered the next time you go to that city during a battle. His special abilities are made up of his limit break special attacks from "Final Fantasy VII." These attacks need time to charge, similar to spells, so enemies can walk out of range on occasion. This ends up making his abilities less useful than most characters.
Balthier
Balthier is the only new secret character added in the remake version, Final Fantasy Tactics: The Ware of the Lions. He appears in the Dorter Slums after you read the "Rash of Thefts" rumor in a tavern. He is one of the more powerful characters available. His Sky Pirate class contains all of Mustadio's special abilities, all the Thief Steal abilities and Barrage. Barrage effectively allows him to do double his normal weapon damage every turn with no repercussion. Equipped with a good gun, Balthier can kill or cripple enemies almost anywhere on the battlefield with ease.