Masayoshi Tsuruda, who goes by the nickname Jumbo, is a doctor and the central figure of Town Doctor Jumbo!!. He returns to a small port city where he once worked, having left twelve years earlier after becoming involved in some kind of trouble. His return is prompted by a loan he had given to the late director of Baba Clinic, a debt that allows him to claim ownership of the clinic and install himself as its new director.

On the surface, Jumbo presents himself as greedy, brutish, and arrogant. He wields the loan and the deed to the clinic as leverage, and his blunt, high-handed manner immediately puts him at odds with the clinic's staff, especially the late director's daughter, nurse Asuka Baba. Despite this abrasive exterior, Jumbo is an exceptionally skilled and perceptive physician. He has a sharp ability to diagnose difficult-to-treat illnesses that other doctors overlook, and he consistently considers the patient's position and circumstances above all else. This genuine dedication to patient care becomes increasingly apparent as the story unfolds, revealing that his gruff demeanor masks a deep commitment to his work.

Jumbo's primary motivation for returning is tied to the loan and the clinic itself, but his actions reveal a more complex drive to serve the community he once left behind. His key relationship is with Asuka Baba, who initially resents his takeover of her father's clinic and is stunned by the change in his character from the person she once admired. As they work together, they clash frequently, but Asuka gradually comes to see his true nature as a dedicated doctor. Jumbo also has ties to the late director, Asuka's father, and the history of their past collaboration and falling out informs his present actions.

Development for Jumbo involves a gradual revelation of his deeper qualities beneath the rough shell. He does not dramatically soften but rather allows his competence and compassion to speak for themselves, earning the grudging respect of those around him. His notable abilities include his unconventional diagnostic methods, which often involve a deep understanding of a patient's lifestyle and habits, and his willingness to take on cases that others deem hopeless. He is also physically imposing and rough around the edges, a contrast to the typical image of a doctor, but this only underscores his effectiveness as a practitioner.