Supreme Commander

Summary
A Supreme Commander is the overall commander of a particular fleet; in the Halo Graphic Novel,

Rank
the armor is depicted as clad in bright violet armor, with a long cloak that wraps around his shoulders. This particular Supreme Commander was a friend and superior to Rtas 'Vadumee, and would later become the Arbiter. Interestingly enough this same Commander wears gold Zealot armour in his trial at High Charity. This change in armor could be assumed the outcome of simply not being a Supreme Commander anymore. The future Arbiter was still a Ship Master prior to and during his trial before the High Council and thus still wore the armor of his rank. So Far only one has been seen. During The Civil War of the Covenant, it is possible the Brutes usurped these positions.

Trivia

 * Supreme Commanders are probably the Covenant equivalent of Vice Admirals and Admirals/Lieutenant Generals and Generals.