Jeffery is the identical twin brother of Jason. He first appears in New Kids On The Block alongside Jason and their mother. He and Jason later join Caillou's playschool class.
In the first couple seasons, both twins wear identical red shirts with a white stripe. In later seasons, Jeffery keeps the red shirt, while Jason wears a reverse of the shirt; white with a red stripe.
Personality wise, the twins act pretty much similar, though Jason once refers to Jeffery as a 'tattletale'. At first, both act mischievous, mainly in the episode Shadow Play, where they fool around and accidentally squirt mustard all over Caillou's hotdog, then both acting like nothing happened. Later, when paying hide and seek, they hide inside rather than outside. When Caillou gets scared, they lightly make fun of Caillou by asking if he's a 'scardey cat'. However, the boys are all still friends by the end. In Cailloodles, the twins act a bit bully-like, calling many classmates 'funny' nicknames (such as calling Clementine a 'goofy swoofy head', Leo 'loop-de-loop Leo, Caillou 'Cailloodles' etc) , much to the annoyance of the others. However, they learn their lesson in the end and agree to just use the 'silly' nicknames with each other rather than the other kids.