When you reach level 60 as a Mercenary, you will be able to job change into a Knight.
As a knight you can take on the role of being the tank for the party. You will be at the front taking damage from monsters and dealing damage to monsters.
You will have good defense, enabling you to shield other members from attacks. You will also have the strength to provide consistent damage against monsters.
As a Knight you will wear heavy armor for protection. To fight, you will use swords and axes.