Unlockable Players
Exchange Student: Unlocked by buying in the Modern Dungeon Issue 1 magazine for 250 coins.
Bookworm: Unlocked by buying in the Modern Dungeon Issue 3 magazine for 250 coins.

Default Classes
These six classes are automatically unlocked right when you begin a new game.
01 Cleric
02 Paladin
03 Warrior
04 Mage
05 Thief
06 Hunter

Coins Classes
These three classes can be unlocked for a certain amount of coins by buying them in the Modern Dungeon Issue.
07 Warlock: Unlocked by buying in the Modern Dungeon Issue 1 magazine for 250 coins.
08 Druid: Unlocked by buying in the Modern Dungeon Issue 4 magazine for 250 coins.
09 Psion: Unlocked by buying in the Modern Dungeon Issue 4 magazine for 250 coins.

Unlockable Classes
These two classes can be unlocked by completing various missions throughout the campaign of the game.
10 Ninja: Unlocked by completing the quest Ninjain' Around (Lv. 7) in Spawn Point Village. (complete Nearby Cave a second time) [unlocked by default with my mod]
11 Barbarian: Unlocked by completing a quest "Fight an arranged battle" (Lv. 20) in in the Red Waste. (defeat Ahnold in the red wastes) [unlocked by default with my mod]

Adventure Classes
These two classes are unlocked as part of adventures, which can be purchased for coins by buying them in the Modern Dungeon Issue.
12 Monk: Unlocked by completing a quest "Travel" and "Escort" (Lv. 25) in the monastery. (you need the Fist of Fury +1 DLC) [unlocked by activating dlc1 in the Modern Dungeon Issue 2: Fist of +1 Fury with my mod]
13 Knight: Unlocked by completing a quest "Fight" (Lv. 28) in "The Great Paywall" and "Meadows".
You will encounter the Knight as a Miniboss at The Great Paywall, once he is defeated a sidequest should pop up with a requirement of level 28, after his second defeat a sidequest will pop up in the Meadows
(you need the Here Be Dragons DLC) [unlocked by activating dlc2 in the Modern Dungeon Issue 3: Here Be Dragons with my mod]