The story is told from Rose's point of view. We learn this is her first solo train trip and that Flo has warned to be wary of what she calls "white slavers" or men who might try to seduce her. All of this indicates that Rose is young and relatively innocent, sheltered and vulnerable enough for Flo to worry about her.
Rose's thoughts reveal that she is in high school, as her mind wanders to her French teacher, Mr. McLaren. When the minister puts his hand on her thigh underneath his newspaper, which overlaps her leg, it crosses her mind with some disgust that he is older than her late father, which again reveals she is youthful.
Rose is not very sophisticated. This is the first time she has had an encounter of this sort, and while it repels her, she is also young and naive enough to be curious about what is happening to her (even a little excited), while at the same time ashamed over it. If she were older and more sophisticated she no doubt would have handled the situation more assertively, pushing the man away in no uncertain terms. But now, as a youngish and curious teenager taught to respect authority, she doesn't want to confront this older man.
Munro uses Rose's thoughts and actions to reveal things about her, rather than having an objective narrator explain her to us.