Final Fantasy Tactics A2: Grimoire of the Rift/Secret Characters

Frimelda
By following a chain of quests starting with "Under the Weather" (B4-07) and ending with "The Way of the Sword" (B5-11) the female paladin turned zombie will join your group. She looks and acts as a Hume (can go into any Hume class) and comes with some basic paladin action abilities and the Dual wield passive ability.

Monteblanc
You need to be post-game (GAME COMPLETE file) and need three things: 1. Giga's Pendant (quest: "Camoa Cup") 2. Pin of Order (quest: "Fluorgis Cup") 3. Earrings of the Dead (quest: "The Last Step")

Then, there should be a mission available called "Wanted: Friends, Kupo". Do it, then you will unlock the moogle montebanc, who is a black mage class. He knows the full black magic set.

Al-Cid
Al-Cid can only be unlocked after the game has been completed. Then there should be a mission called "A Refined Recruit", and Al-cid will join your party. Al-Cid is a hume Agent, and can only learn Reconnaissance.

Penelo
Penelo can only be unlocked after the game has been completed. Then there should be a mission called "An Elegant Encounter"

Vaan
Vaan can only be unlocked after the game has been completed. Then there should be a mission called "Where Could He Be?"