Bertha is a supporting character who appears as the elderly caretaker and companion to a frail, older woman living in the seaside town that serves as the story's primary setting. She is depicted as a kind-faced woman of advanced age, with brown hair typically tied back and dark eyes. Her role, while small, provides a meaningful connection to the past and offers quiet encouragement to the protagonist during a moment of doubt.

In terms of personality, Bertha is consistently friendly and warm-hearted toward everyone she encounters. Despite her own advanced years, she is devoted to caring for the elderly woman she serves, responding to calls for assistance with nearly every task and ensuring the house and its resident are looked after around the clock. This dedication reveals a deeply selfless and responsible nature. A more whimsical side of her personality emerges in her fascination with flying. Much like the enthusiastic boy Tombo, Bertha is captivated by the idea of flight, and she has secretly imagined herself soaring through the air on a broomstick.

Bertha's primary motivation appears to be a quiet yearning for connection and a sense of wonder that has not faded with age. Her care for the elderly woman is not merely a job but a genuine commitment born of affection and duty. However, her secret fantasy of flying suggests a personal desire for freedom and adventure that her daily responsibilities do not provide. This makes her an unexpected kindred spirit to the young witch, who represents the very magic and independence Bertha dreams about.

Within the story, Bertha functions as a customer and a subtle mentor figure. When Kiki first arrives to make a delivery, Bertha is delighted to welcome her into the house. Unlike many townspeople who view Kiki with suspicion or indifference, Bertha has a personal understanding of witches, as her own great-grandmother was one. This familial connection allows her to see Kiki not as a curiosity but as a person continuing a respected tradition. This acceptance provides Kiki with a small but significant sense of belonging at a time when she often feels like an outsider. Her relationship with the elderly woman she cares for is one of mutual dependence, though Bertha is the primary source of physical help. The older woman relies on her for almost every task, from changing lightbulbs to daily chores, a responsibility Bertha carries with patience even as her own age makes such work difficult.

Bertha does not undergo a dramatic personal transformation over the course of the narrative, but her presence contributes to the development of the central theme of finding magic in everyday life. She and the elderly woman prepare a special herring and pumpkin pie for the old woman's granddaughter, a gift of love that becomes crucial to the plot when Kiki loses her flying ability and must deliver it on foot. This act underscores Bertha's role in creating moments of heartfelt connection. Her notable abilities are not magical but practical and emotional. She is an exceptionally devoted caretaker, a skill that requires immense patience and physical endurance. More importantly, she possesses the ability to nurture wonder in herself and others, a quality that proves just as valuable as any spell. Her secret act of sitting on Kiki's broom and pretending to fly is a small but powerful display of imagination, showing that the desire for magic and flight is not limited to the young.