Caillou (translated pebble or stone in French and nicknamed The Prince of Imagination) is the title character of the show. Caillou was first voiced by Bryn McAuley from 1997 through 2000, then Jaclyn Linetsky in 2000 through 2003, and then, due to Jaclyn Linetsky's death, Annie Bovaird from 2003 through 2009. Caillou was first shown in the episode "Caillou Makes Cookies", which aired in 1997.

Caillou is an average, imaginative four-year-old boy with a love for forms of transportive machinery such as rocket ships and airplanes. A dreamer, Caillou is prone to frequent dream sequences in some episodes visualizing his daydreams and hopes, and many episodes chronicle his normal daily experiences with his parents, friends, and neighbors. Caillou particularly loves his stuffed dinosaur Rexy and teddy bear Teddy, along with his pet cat Gilbert, all of whom are depicted as puppets in segments featured in the earlier episodes.