Paladin Class Unlock - Dragon Quest 9 Guide
Quest Requirements: Use Whipping Boy to protect a party member 10 times
Reward: Paladin Vocation Unlocked
The quest to unlock the Paladin class is found in the Mirage Mahal, the largest building in northern Gleeba. Ontop of this building you'll find a man standing just above the fountain with whom you'll want to speak to. Use my screenshot above if you need additional guidance.
Paladins are a mix of the Priest and Warrior class in terms of their abilities and spells. They have some of the highest HP and defense in the entire game as well making them by far the most durable class with heal spells.
For the quest to unlock the Paladin all you have to do is use the Whipping Boy skill to protect a member of your group a total of 10 times while you're fighting enemies in the Djust Desert. In order to unlock the Whipping Boy ability you'll need to level up the Warrior class on a member of your team and put at least 8 skill points into Courage.
If you'd like to make this quest a buttload easier you could level up multiple Warriors and use Whipping Boy on multiple characters each round, almost guaranteeing you get credit each turn. Honestly though this is probably the easiest class unlock quest of them all - you don't have to cheese it that much to do it.