Knight Online has four character classes for available both nations: Warrior, Rogue, Mage, and Priest. Each class has a basic skill set, three main skill sets (Level 10 and up), and one master skill set (Level 60 and up).

Armor Classes
Tier 1: Also known as "Paper". The paper build sacrifices armor defense for higher attack. A character focuses its stat point distribution so that only its attack goes up. Later in the game, a paper character would try to obtain other items to increase defense such as unique jewelry. Only mages and battle priests use this build as rogues and warriors use plate because they use plate for their "paper" builds.
Tier 2: (Half-Plate) A very uncommon build. The only reason that it would be used is because half-plate is cheaper to upgrade (compared to Plate). With a higher + item you can get more of the stat you want to do more damage (dexterity for rogue and strength for warrior).
Tier 3: (Plate) Common for lower level rogues and warriors. It is like the paper build for mages and priests but instead for warrior and rogue classes. As you grow to a higher it will be seen less and less.
Tier 4: Known as "Full Plate." This is a popular aim amongst high-level paper characters. Because of the maximum stat distribution cap of 255, paper characters can begin to focus on secondary stats to increase armor capability. Full Plate is also great because of its availability to obtain and relatively low cost.
Tier 5 and Tier 6: (Chitin and Chitin Shell) These armors give the most defensive ability. The difference is that Chitin Shell is reserved for Master characters, and it also provides more defense.