is a 4 year old who is one of Caillou's friends. He has a twin brother named Jeffery. Jason and Jeffery first appeared in Season 2 in the episode New House, New Neighbors. Back then he and Jeffery both wore identical clothes (red shirts with a white stripe and pants) but, in Season 4 Jason began to wear a white shirt with a red stripe and they are now able to be told apart. In the episode Caillou the chef , Jason has a toy spider named Boris. Towards the end of Season 2 he and Jeffery began coming to Caillou's preschool class. In Seasons 2 and 3, he and Jeffery appear to be mischievous and sometimes a little mean to Caillou but, in Season 4, they are nicer to Caillou and more of kind people. He is voiced by Adam Mover.


Note: Although Jason and Jeffery are twins they don't appear in all the same episodes Jason doesn't appear in the episode that Jeffery appears in Garage sale and Jeffery doesn't appear in A surprise for Ms.Martin

Caillou's Playschool class
Caillou Clementine Emma Jason Jeffery Leo Xavier