The detail that helps you understand that Rosemary is a complex character is: “Be frightfully nice to her. Look after her. I don’t know how. We haven’t talked yet. But show her—treat her—make her feel——”
This line suggests that there is more to Rosemary than meets the eye, as the speaker feels a strong need to care for her despite not fully understanding her. It indicates that Rosemary may evoke strong feelings in others and may have a depth that is not immediately visible, suggesting her complexity.