Swordsman, Knight, Archer, Thief and Sorcerer are unlocked from the start.

Pirate, Adventurer, Half-Beast, Ranger, Wanderer and Ult. Student are unlocked from the start but must be purchased with Dream Stones.

Other classes are unlocked either by completing NPC quests or by beating the game with other classes. Please see below for unlock requirements:

One Way Hero
-Clear a Campaign World to unlock the King Victor NPC.
-Place King Victor in your castle and clear his quest on [Normal] difficulty or above to unlock the class.

Dracon
-Clear two Campaign Worlds to unlock the Well Read Resident NPC.
-Place the Well Read Resident in your castle and clear their quest on [Normal] difficulty or above to unlock the class.

Wrestler
-Clear 3 different Campaign Worlds to unlock the Head Chef NPC.
-Place the Head Chef in your castle and clear their quest on [Normal] difficulty or above to unlock the class.

Samurai
-Clear a Campaign World as 3 of the 5 starting classes to unlock the Wounded Samurai NPC.
-Place the Wounded Samurai in your castle and clear their quest on [Normal] difficulty or above to unlock the class.

Beast Hunter
-Clear a Campaign World as a Knight, Ranger and Half-Beast to unlock the Skilled Hunter NPC.
-Place the Skilled Hunter in your castle and clear their quest on [Normal] difficulty or above to unlock the class.

Astrologist
-Clear a Campaign World to unlock the King Victor NPC.
-Place the Fortuneteller Pupil in your castle and clear their quest on [Normal] difficulty or above to unlock the class.

Priest
-Clear a Campaign World as an Astrologist or as 17 different classes.
-You can now purchase the class for 150 Dream Stones.

Gunner
-Clear a Campaign World as an Archer or as 3 different classes.
-You can now purchase the class for 100 Dream Stones.

Brawler
-Clear a Campaign World as an Adventurer or 5 different classes.
-You can now purchase the class for 150 Dream Stones.

Assassin
-Clear a Campaign World as a Thief or as 9 different classes.
-You can now purchase the class for 150 Dream Stones.

Zombie
Clear a Campaign World as 12 different classes.
-You can now purchase the class for 300 Dream Stones.

Queen
Clear a Campaign World as 12 different classes.
-You can now purchase the class for 300 Dream Stones.

Dark Knight
Clear a Campaign World as 12 different classes.
-You can now purchase the class for 300 Dream Stones.

Lord
-Clear a Campaign World as a Dark Knight or Queen or as 14 different classes.
-You can now purchase the class for 600 Dream Stones.