A USAian is a citizen of or resident at the United States of America, can also be used to describe a non-US citizen but loyal to the USA. The same term can be used for agents hired by one of the United States agencies or corporations and reaching the level of ‘Pleased by Masters’.
USAese can also be used, but it’s mainly used for the same as above but of oriental origins like people coming from China and Japan ‘evolving’ from a Chinese or a Japanese to a USAese.