Jason Marsden (October 14, 2001) is Caillou Anderson's best friend in Caillou Gets Grounded and it's Sequel Caillou Gets Ungrounded. He's a bit of a smart aleck, but is much more intelligent than Caillou. He has trouble attracting women due to his small stature. He is voiced by Jason Marsden in Caillou Gets Grounded and Caillou Gets Ungrounded and by Hal Sparks in Caillou Gets Ungrounded.
- In Caillou Gets Grounded: Caillou Gets Eaten By Piranhas Caillou leaves him for Vicky and she pays Jason to feed Caillou to the Piranhas.
- Went driving with Caillou despite the fact that neither of them had their license in Caillou Gets Grounded Gets Boris' Car Crushed.
- Tried to dissuade Caillou from dating Desiree in Caillou gets dumped by his girlfriend Part 1 and Caillou gets dumped by his girlfriend Part 2 but ended up dating Bella (From Bella and the Bulldogs) after they split. But he said he was only going out with her for the break-up kiss.
- He is planning a huge trip to Europe in the summer with Caillou in Caillou Gets Grounded: Caillou Cheats On His Test.
- In Caillou Gets Grounded: Caillou Iceskates he made a tape for Caillou that taught him how to ice skate in his sleep. Because the tape was originally about dieting, Caillou became obsessed with that also.
- Jason also tag along with Caillou and his dad Boris to go to New York City for the balldrop for the New Year of 2018 in Caillou Gets Ungrounded: Caillou's painful New Year.
- Jason also gets tired of Andre and leaves in Caillou Gets Ungrounded: Caillou stands up for his friend.
- Jason appears in Caillou Gets Ungrounded: Caillou's Relationship Drama where he and Sausage fights over Whitney.
- Jason did not appear in Caillou Gets Ungrounded: Caillou Gets Engaged but he is mentioned by Caillou that he went to a Comic Con and meets Spider-Man.
- In Caillou Gets Ungrounded: Caillou Stops The Meltdown Jason Caillou Whitney And Jack Griffo stops the meltdown that Brittney tried to destroy the world from.
- In Caillou Gets Ungrounded: Rosie's Valentines Day Goes Wrong Jason cheers up Rosie after being dumped by Andre.
- In Caillou Gets Ungrounded: Caillou Meets The Queen Jason goes with Caillou Whitney Doris and Boris to meet the Queen but Boris ends up getting all of them kicked out.
- In Caillou Gets Ungrounded: Caillou's Wedding Day Jason and Boris were talking and hanging out while Caillou and Whitney got married.
- In Caillou Gets Ungrounded: Caillou's Honeymoon Jason appears talking to his Girlfriend but when Caillou and Whitney appeared neither one of them realized it was him.
- In Caillou Gets Ungrounded: Caillou Gets Kicked Out Of The Restaurant Jason goes with Caillou and Whitney at a Restaurant but gets kicked out after Caillou burps at the waiter.
- He was the only character on the series to have the same exact name of the actor portraying him.
- Jason's birthday (October 14, 2001) is the same day that Actress Rowan Blanchard was born.
- Jason is also voiced by Hal Sparks as Jason Marsden's Understudy marketing the third character that Sparks replaced Marsden the first two were Tak and the Power of JuJu television series based on the video games as Tak and the other Fuller House as Nelson Burkhard from Full House.
- Jason wears a red shirt when he is voiced by Jason Marsden and wears a blue shirt when he is voiced by Hal Sparks.