Diana, originally Diana Johnson, enters the story as the second wife of Frederick MacDowell, becoming the stepmother to his daughter, Jeanie. She comes from a wealthy and successful business family, a background that initially makes her unfamiliar with the simpler lifestyle of the MacDowell household. Her connection to the family begins when Frederick saves her sick father's life, an act that leaves her feeling deeply indebted to him and ultimately leads to her falling in love with him. Despite her own father's disapproval of the match, as Frederick was not a wealthy man, Diana chooses to marry for love, willingly leaving her privileged life behind.

Personality-wise, Diana is portrayed as spoiled sweet. While she is used to a life of wealth, she has a fundamentally kind and generous nature. This is demonstrated early on when she surprises Jeanie with a large birthday cake and many gifts as a way to thank Frederick for his help. She is a determined woman, inspired by her husband's honesty and generosity to help the sick and needy, which motivates her to begin training as a doctor herself. Her primary role in the story is as the new matriarch of the MacDowell family following the death of Jeanie's mother, Angela. She strives to be a part of the household and to build a relationship with Jeanie, though this proves to be a significant challenge.

Her relationship with her stepdaughter is the most complex and fraught aspect of her role. Diana does not hate Jeanie, but their bond is tense because Jeanie feels that Diana is trying to replace her deceased mother. This makes Diana a reluctant guardian figure, as her desire to be close to Jeanie is met with resistance and resentment from the grieving girl. Her relationship with her husband, Frederick, is a loving one, built on admiration and respect for his character. She also understands and respects the memory of his first wife, Angela, accepting her role as a second love rather than a replacement.

In terms of development, Diana starts as an outsider from a wealthy background who feels indebted to Frederick. She evolves by embracing her new life, working to gain acceptance from her stepdaughter, and pursuing medical training to follow in her husband's footsteps. A notable ability she acquires during the story is her training to become a doctor, a path she undertakes for Frederick's sake and inspired by his example of caring for others.