To unlock them, you need a concrete adventurer to be raised to level 10.

Job	How to unlock
Archer	Raise 'May Le Tigre' to level 10.
Bard	Raise 'Seffy Roth' to level 10.
Black Marketer	Raise 'Voldy Moore' to level 10.
Clown	Raise 'Flippin Book' to level 10.
Cook	Raise 'May Akiba' to level 10.
Hero	Raise 'Harly Potler' to level 10.
Kairobot	Promote your village to 5 stars.
King	Raise 'Sam Ganges' to level 10.
Knight	Raise 'Gilly Gamesh' or 'Jill Nabath' to level 10.
Kung Fu Master	Raise 'Chung Lailai' to level 10.
Mage	Raise 'Harry Gorn' to level 10.
Mercenary	Raise 'Frod Bigguns' to level 10.
Monk	Raise 'Stumblemore' to level 10.
Ninja	Raise 'Book Shields' to level 10.
Princess	Raise 'Lily Raven' to level 10.
Princess Sally	Promote your village to 5 stars.
Warrior	Raise 'Donkey Hotty' or 'Angie Jelly' to level 10.
Wizard	Raise 'Max Spellman' to level 10.
Wrestler	Raise 'Masque Mann' to level 10.