He has fair hair and blue eyes. He is left-handed, which tended to get him into trouble at school.
He is prone to car-sickness.
As a child, Cat lived with his parents and sister upstairs at No. 26 Coven Street, Wolvercote. After their parents were killed in the Saucy Nancy Disaster, they were taken in by Mrs Sharp, who lived downstairs at No. 26, and the town of Wolvercote undertook to pay for their education and upbringing.
He was nervous and clinging, and tended to follow Gwendolen's lead in everything. Unlike her, he has no magical ability to speak of.
After the Fortune-Teller a few doors down tells Gwendolen's fortune, Miss Larkins the Clairvoyant insists on telling Cat's. She goes into a trance and speaks with a man's voice, which tells Cat that "there'll be a big change coming up for you now", but that "you've been awfully careless - four gone already, and only five left" and that "you're in danger from at least two directions". Miss Larkins is as puzzled by this as Cat, particularly since she's never spoken with a man's voice before and has no idea who it was.
Not long after that - about a year after their parents' deaths - Cat and Gwendolen are taken to live at Chrestomanci Castle with Chrestomanci and his family (who are cousins of theirs, although it is some time before anybody thinks to explain this).
See also Charmed Life (spoilers).