What’s he highest military rank in your world?
In the Imperial Armed Forces(of Japan)/IAF, the highest rank is Imperial General, formally known and addressed as[the]Imperial General of the Armed Forces of Japan. The Imperial General is the supreme commander of all 3 branches of the Imperial Armed Forces(the Imperial Army of Japan, Imperial Navy of Japan, and Imperial Trooper Corps), and has heavy influence in the National Police Corps(NPC). During times of extreme national emergency, the Imperial General may be given command of all NPC police officers. Excluding the NPC police officers, the Imperial General is the commander of over 15,000,000 soldiers, sailors, and Troopers, and has a direct and final say in military matters. When you factor in the NPC(during extreme times of national emergency of course), the Imperial General commands over 20,000,000 personnel throughout all of Japan.
To receive the rank of Imperial General is an extremely gracious honor that automatically assumes absolute respect within the military. It's the greatest sign of honor to obtain the rank, and it shows ones devotion to service to the Emperor and Japan. It's considered the 3rd most important "societal rank" in society, just behind the Prime Minister and Emperor(2nd and 1st respectively). In fact, if he/she wants to, the Imperial General may even launch a coup against the Prime Minister or even Emperor. However, the Imperial General must display respect to his superiors(the Emperor, Prime Minister, etc.), thus launching a coup/rebellion/etc. would be useless.
The rank is such a prestigious honor that even regular citizens come to attention when they hear about the Imperial General. The rank may only be given to someone with decades of raw military experience. It may only be given to one soldier, sailor, or Trooper(who's an officer with at least 20-25 years of leadership/service)once every 7 years(the Imperial General may hold the position for 2 terms/14 years, but then they must retire), and only the most experienced, loyal, dedicated, and honorable of men and women may obtain the rank. They must also excel and have extensive in both conventional/human-human warfare as as anti-Red Corpses warfare. Due to the importance of the supreme commander of all the branches of the military, a company of 75 elite soldiers and Troopers is dedicated to be the Imperial General's security detail. They guard the residence of the Imperial General(Noji Castle), and it's considered an extremely high honor to guard the Imperial General.
Currently(1927), the Imperial General is Imperial General Fuharuto, the son of the former and glorious Imperial General Fukushiro. Imperial General Fuharuto is also the youngest Imperial General in Japanese history, being only 49 years old).