C.J. is first introduced when Brian befriends him on the train towards "The Perfect Boarding School". He tells Brain that he was also sent to this school amend his behavior. When they finally arrive, the two are then separated as Brian is sent to a much more advanced course called "The Pattern Room". Throughout the story, C.J. is never seen as he's shown to go through the protocol that most of the students go through in the school. That is until we see him later on in the story attempting to stop Brain from escaping.
It's revealed that C.J. works for the Guardians though it's unknown if C.J. is supposed to be a robot or just an evil kid that is just obeying the Guardians. Nevertheless, Brian defeats him and is able to lock both him and his robotic look-alike in some cells below the school. The status of whether or not that C.J. was able to get out of his cell is unknown.
In the short story, he's appearance ranges to a Caucasian kid with blonde hair and a black and white polka-dot shirt. But in the TV adaptation, he's portrayed as an Asian kid with black hair and a brown coat with a tie and white undershirt.