The job of the Apprentice is to guard the Leader and train with the Leader, with hope to one day become the Leader.
In war, the Apprentice battles side by side with the Leader, threatening to kill all who trespass and come close to the Leader. The Apprentice also guards the Leader even off the battlefield, beside his throne.
The Apprentice is also trained by the Leader personally. When the Leader dies, the Apprentice becomes the Leader. However, the Leader has never died, so no Apprentice has ever become a Leader.