Calvin is an impulsive, sometimes overly creative, imaginative, energetic, curious, intelligent, and often selfish six-year-old, whose last name the strip never gives. Despite his low grades, Calvin has a wide vocabulary range that rivals that of an adult as well as an emerging philosophical mind, implying that he comes from a naturally literate family.

He has also said, "You know how Einstein got bad grades as a kid? Well, mine are even worse!" He commonly wears his distinctive red striped shirt. He is also a compulsive reader of comic books and has a strong thirst for pop culture, including trendy or modern apparel, pulp fiction, and electronic devices—which mostly remain a wish throughout the series. Nevertheless, his often-vulgar experiences seem to give him valuable experiences, contrasting his preferences (and unusual derivations of thoughts) to the standards and morals his parents set to him. Calvin chews gum regularly and subscribes to a magazine called "Chewing". <a href="http://fumbbl.com/FUMBBL.php?page=player&player_id=3785825">Hobbes</a> occasionally challenges Calvin's strange obsession, but otherwise just remains calmly waiting for the strip to end. Throughout the series, he is also revealed to be a "try and err" sort of person.

On many occasions, Calvin sees himself in one of his many alternate guises: as the astronaut and explorer <a href="http://fumbbl.com/FUMBBL.php?page=player&player_id=3785834">Spaceman Spiff</a>, the superhero <a href="http://fumbbl.com/FUMBBL.php?page=player&player_id=3785857">Stupendous Man</a>, the private eye <a href="http://fumbbl.com/FUMBBL.php?page=player&player_id=3785837">Tracer Bullet</a>, and many others.