Ralph functions as the cosmopolite who has interested himself in Isabel's career. Upon first meeting her, he senses her potential capacity for development. He then devotes his life to observing Isabel's activities.
Ralph is well suited for his function in the novel. Knowing that he has only a few more years to live, he has developed a sensitivity which enables him to penetrate to the center of things. Owing to his sickness, he can remain relatively uninvolved and objective. But he is not completely disinterested. He feels that his last remaining years will be enriched by observing Isabel's activities. Thus, he is instrumental in placing a large fortune at her disposal. His act was performed in order to provide Isabel the opportunity to develop to her fullest capacity. But in actuality, it caused her to become a prisoner.
Ralph's objectivity and sincere love for Isabel also allow him to function as a confidant to her. He is able to discuss intimately with her various aspects of her career and thus give the reader a more rounded view of any situation.
Ralph, then, is a person of high intelligence and sensitivity who is able to perceive the essential aspects of any situation, and the reader has access to what he knows. Ralph knows that Osmond is a "sterile dilettante," but is unable to convince Isabel of this fact. Furthermore, he recognizes that some people, like Madame Merle, are too perfect. He remains, however, Isabel's closet friend and admirer and the person who brings about her first open break with Gilbert Osmond.