Calvin, as one of the two title characters of the strip, is instrumental in its actions. He appears in almost every single strip (yes, he is missing from a few), and is the focus of the strip's stories and humor.

In order to keep the characters ageless, Calvin's birthday is left unknown. He is six years old and attends first grade in an unnamed school. Calvin is named after John Calvin, a leader in reformation of the Catholic Church.

The comic is fueled by Calvin's imagination, which appears to be unending. He has appeared more than once as Tracer Bullet, Stupendous Man, and the great Spaceman Spiff.