Raymond Abrashkin died after the fifth book. Williams, however, insisted on Abrashkin being given co-author credit on the subsequent books as well, since he had been instrumental in constructing the series. Ezra Jack Keats illustrated the first four novels in the series.
Professor Euclid Bullfinch, a researcher at the fictional Midston University. The professor is also a musician who plays the bass viol (also known colloquially as the "bull fiddle"). He is plump and somewhat bald.
Teenager (some books depict an elementary school setting) Danny Dunn, who gets into complicated problems involving the professor's latest invention. Danny is looking forward to a career in science, and admires Professor Bullfinch as a mentor.
Mrs. Dunn, Danny's widowed mother, housekeeper for Professor Bullfinch.
Teenager Irene Miller, Danny's friend and next-door neighbor. Irene's father teaches Astronomy at Midston University. Irene is particularly interested in physics.
Teenager Joe Pearson, Danny's friend. Joe is the poet of the group, and often functions as an amusing sidekick to Danny, expressing bafflement at the complicated technology employed by Danny and the Professor.
Teenager Eddie ("Snitcher") Phillips, rival of Danny
Doctor A.J. Grimes, a friend of Professor Bullfinch introduced in the first book, Danny Dunn and the Anti-Gravity Paint. Grimes is a curmudgeonly figure, rarely taking the teenagers seriously, and often trying to antagonize the Professor. Doctor Grimes is also a musician who plays the piccolo; he and Professor Bullfinch occasionally play duets. Tall and lanky, he is in many ways a contrast to his friend Bullfinch.