There are six bonus classes you can unlock in Knights of Pen and Paper on the Android and iOS systems (iPhone, iPad, etc.).
You don’t have to even beat the game to unlock them, you just need to complete specific quests at certain locations in the game.
Here are the bonus classes you can unlock, and how to unlock them.
Barbarian - Unlocked from a quest involving a delivery to Goblin’s Hideout that starts in Journey Village.
Bard - Complete the Rescue Princess quest at Sunset Castle, then complete the Slay Bandits quest. You’ll be sent off to find the Bard after you defeat the Bandits.
Hunter - Complete the collection quests for Red Apples, Green Apples, and Carrots in Default Village.
Knight - Complete the Battle Skeleton quest at Sunset Castle.
Necromancer - Complete the Escort Elder quest in Default Village.
Shaman - Complete the Travel to Plains Temple quest at Yoga Village, then complete the quests available at Plains Temple.