The primary classes on Zodiac are Mage, Monk, Priest, Scout, Thief, and Warrior. Once you level up two of these, they combine and become a "hybrid class". The combinations for the hybrid classes are as follows:
- A Level 10 Thief and Level 10 Warrior gives you an Assassin class.
- A Level 10 Thief and Level 10 Mage gives you a Ninja class.
- A Level 10 Thief and Level 10 Scout gives you a Ranger class.
- A Level 10 Priest and Level 10 Mage gives you a Summoner class.
- A Level 10 Scout and Level 10 Priest gives you a Druid class.
- A Level 10 Warrior and Level 10 Mage gives you a Dark Knight class.
- A Level 10 Warrior and Level 10 Monk gives you a Gladiator class.
- A Level 10 Warrior and Level 10 Priest gives you a Paladin class.
Sadly there are no stage 3 classes or anything like that as far as I'm aware (haven't played in a while) so it's really up to you which direction you want to go in a secondary class.