I am nominating Antoinette Conway. She is canonically mixed-race; [SPOILERS] while she does meet her biological father, who is nonwhite, she ends up rejecting his offer to restart a family relationship, and thus we never find out his exact name or nationality. That said, she is decidedly not white passing and there's a plot component to it too, as she deals with a lot of harassment within her squad and it's generally implied that part of that is due to her being the only nonwhite person on the squad.