Roberta is the eldest of the railway children and is already the most thoughtful and introspective of them at the beginning of the book. She understands more of what has happened to the family than Phyllis and Peter. She is also closer to their mother and more concerned to protect her from being hurt. However, despite her intellectual and emotional maturity, Roberta has led quite a sheltered life and has to grow up in numerous ways, primarily in adapting to the family’s comparative poverty and realizing its implications. For instance, when their mother falls ill, Roberta tells the doctor, with considerable reservations, that they are poor and asks whether they could join a club, such as the one Mrs. Viney told her about, to spread the cost of treatment.

Roberta is described as an exceptionally helpful person who is more than usually concerned for the welfare of others:

Bobbie had another quality which you will hear differently described by different people. Some of them call it interfering in other people's business—and some call it "helping lame dogs over stiles," and some call it "loving-kindness." It just means trying to help people.

This is a quality which develops and becomes more active in Roberta’s character over the course of the book, particularly in her determined and ultimately fruitful efforts to help the Russian gentleman, Mr. Szezcpansky. While she becomes more practical in her altruism, Roberta also grows increasingly self-effacing and aware of her own reactions. She does not want the Director of the Railway to tell her what he thinks of her, imagining that she will be embarrassed, and when she tells Mr. Szezcpansky that his wife and children have been found, she wishes that she had not carried the news, as the emotion is too intense to handle.