Supreme Commander

The title of Supreme Commander is a rank given to the most senior officer in the Imperial Armed Forces of the Imperial Kingdom, including Maynor Hajime, Amdis' "son". Although the Emperor, being the overall leader of the Imperial Kingdom as head of state, head of government and commander-in-chief, held absolute authority over the Imperial Kingdom, he did not hold direct command over the organizations and branches that exist within the Imperial Armed Forces, which instead went to the Supreme Commander.

The role had been given to several individuals over the course of the Kingdom's life cycle. Both Kin and Adrian had appointed military officers as their chosen Supreme Commanders, but Amdis did not follow their example and instead appointed his apprentice Darth Vader as the Supreme Commander.

Powers
The Supreme Commander is considered to be one of the highest honours to be given by the Emperor by many people within the Imperial Kingdom, mainly officers of the Armed Forces. The position and title also appears to be the second-in-command of the Imperial Kingdom, as Maynor is able to directly take command of Imperial High Command. As such, the Supreme Commander is the first in line to become the Emperor should Amdis become incapacitated or is killed, however since the Emperors were secretly Sith Lords, that has happened never occurred.