Tom's father, a skilled inventor and businessman, provides Tom with guidance, resources, and moral support. He is a constant presence in Tom's life and often collaborates with him on his projects.

Tom's loyal and resourceful African-American employee and friend. Koku is a constant companion and helps with Tom's inventions and adventures. His portrayal, while common for the time the early books were written, reflects problematic racial stereotypes that should be viewed in historical context.

Tom's romantic interest and eventual wife (in some series). Mary provides Tom with emotional support and often assists him in his adventures, demonstrating her intelligence and courage.

Tom's close friend and often serves as a comedic foil and a source of support. Ned is frequently involved in Tom's adventures, although he often finds himself in humorous or precarious situations.

Tom frequently encounters rivals and adversaries who seek to exploit his inventions or sabotage his efforts. These antagonists provide conflict and challenge Tom's skills and resourcefulness. Notable rivals often included wealthy industrialists or foreign agents.