Lieutenant McMillan was an astronaut working for the NASA in the late 20th and early 21st century.

She had undergone training together with Shannon O'Donnell during which she suffered from claustrophobia. Unlike O'Donnell, McMillan remained with NASA and was to be the co-pilot on a joint mission with the European space agency in 2003, during which she'd spend four months on the International Space Station. (VOY: "11:59")