Enlisted Marine Corps ranks are broken down into three levels, Junior Enlisted, Non-Commissioned Officers, and Staff Non-Commissioned Officers. As a Marine moves up the ladder his or her responsibility and contribution to the mission increases.
Also, children born abroad to two non-citizen nationals or to one alien parent and one non-citizen national parent are eligible for non-citizen national status. This is so rare that the State Department does not even issue a certificate for it. Non-citizen nationals are delineated as such in their passports.
Because the U.S. Virgin Islands are a territory of the United States, U.S. citizens do not need a passport to travel to and from the islands. You will still need an ID to fly, as always, but you won't be stopped by customs and will not be required to present your passport when you enter the U.S. Virgin Islands.