Protector is the fifth rank in the Dark Brotherhood, which is awarded after completing the requirements of a Acolyte. It is at this point within the Journeymen class that one may wield a basic lightsaber given to them by their superiors.
It is part of the Journeyman class, the fifth step down the path to Dark Jedi Knight.
A Rollmaster has the authority to promote up to this rank.
The Requirements to be promoted past this rank are much more difficult, and are as follows:
The requirements for the promotion from Protector are found at the above link, the completion of the above requirements will earn a promotion to Guardian.