Supreme Commander

Supreme Commander is a high Covenant Sangheili (Elite) rank.

Rank
The Supreme commander is indeed placed in the position of a high rank generally organized withing the sophisticated and complex Covenant.It is certain to say that the this particular commander has the rightful purpose of commendeering a fleet of a possibly unknown amount of the covenant's well-built and advanced warships.It would be unjust to say that the Supreme Commander is below the Imperial Admiral or repeating over to a conclusion refering to the fact that the Supreme Commander is above the Imperial Admiral. It may be possible that Imperial Admiral is'nt really of the Covenant but is more of a Heretic and rebllion rank but this is also just speculation.For now it is that we must see and play the "which-is-which" game or simply wait until solid proof is released possibly in future novels,graphical novels/comics, or maybe even Halo 3.However,it is in fact quite interestingly possible that the Supreme Commander is below the deadly yet clever and precised Councillor along with the well honored Arbiter (Saying that it is below the Arbiter opposes and challenges against the fact that Arbiter may not really play a military role of commanding othes which also goes along the Saingheili Councillors).

Physical Appearance
When commanding a fleet, Supreme Commanders wear violet armor with a cloak that wraps around and over the armor piece of their shoulders. However, when the Arbiter was being tried for the destruction of Halo, he wore golden Zealot armor, which could signify that Supreme Commander is one of several Zealot ranks, although this has yet to be confirmed.The fact that the Arbiter wore the golden armour of a Zealot yet wore an unknown violet pigmented armour is quite odd,yet it could prove that the creators/creator of that particular story part were exaggerating the Supreme Commander's appearance to allow the story to become a bit more interesting.To prove this theory generally in Sunrise Over New Mombassa the elites and there weapons appear to be exaggerated also.Ex. The plasma swords are pink along with the fact that plasma rifles shoot green beams opposng to the fact that the original plasma rifles seen shoot cyan repeater-type bolts.

Halo Graphic Novel
The particular Supreme Commander in the aforementioned text was a friend of and yet superior to Rtas 'Vadumee, and would later become the Arbiter. During the Civil War of the Covenant, it is possible the Brutes usurped these positions from the Elites.