He appears in every story in the series (except "Warlock at the Wheel", in which he has an off-stage part), usually in a supporting role and usually under the name (or rather title) of Chrestomanci - the sole exception to both of these is The Lives of Christopher Chant, chronologically the first in the series, in which the boy Christopher Chant is the protagonist and Gabriel de Witt is Chrestomanci.
He is tall, and has black hair, which was curly when he was a boy.
Christopher Chant is the son of Cosimo and Miranda Chant. He grew up in a large house in London, raised mainly by a succession of nursery maids and governesses, and saw his Papa so rarely that when he was small he was haunted by a fear that one day they would meet and he would not recognise him.
As a child, he resisted his Mama's attempts to groom him for a place in Society, since the only Society he'd heard of was the Aid the Heathen Society, which did not appeal. He learned better when he went away to boarding school -- the first chance he'd really had to associate with people outside his family circle -- and went on to become as elegant and splendid a member of society as his Mama could wish.
Early in his life, Christopher found he could travel between worlds via astral travel through the Place Between, although to begin with he thought he was only dreaming this. He occasionally managed to bring things back (most of the time, he lost things while scrambling about through the Place Between: which was still better than most astral travellers, who could not touch things in other worlds, let alone bring them back). These included a clockwork dragon, a flute shaped like a horse, a necklace of tiny pearl skulls given him by the mermaids in Series Five, and a set of bells from Series Ten. Meeting the mermaids also prompted him to try and discover whether women in his own world had fish tails under their dresses; this led to the Incident with Lady Badgett.
After The Last Governess recognised the significance of the artefacts he had brought back from other worlds, his Uncle Ralph asked him to do a series of experiments to see just what he was capable of. These experiments were carried out under the supervision of Tacroy. On his second experimental trip, to see if he can bring back an Asheth Temple cat from Series Ten, he met the Living Asheth, an encounter which was to profoundly shape the rest of his life.
Apart from this sign of talent, which only Uncle Ralph, The Last Governess, and Tacroy knew about, Christopher seemed to be unable to do magic at all. At school, he took to getting his friend Oneir to cover for him, while he in return covered for Oneir in algebra.
While he was at boarding school, he became a keen cricketer, being particularly good at bat, and for a while entertained ambitions of playing the game professionally. Then one day, during cricket practice, he was accidentally hit in the head by a cricket bat wielded by Oneir, and killed instantly. The fact that he was right as rain again by nightfall led to the discovery that he was a nine-lifed enchanter. His father took him out of school to be tutored by Dr Pawson, who discovered why he was unable to do magic (which will not be revealed here, as it is both a spoiler for ''The Lives of Christopher Chant'' and a spoiler for ''Charmed Life''), before being taken off to live at Chrestomanci Castle under the guardianship of Gabriel de Witt, who was then Chrestomanci.
Following Gabriel de Witt's retirement, he became Chrestomanci himself.
As an adult, he is extraordinarily well-dressed, with smooth shiny hair. (His hair is still wriggly when it gets wet.) He never seems to be seen in the same elegant outfit twice. Most famous, to the lucky few who have seen them, are his dressing-gowns, which include:
He drinks tea without milk or sugar, but with lemon.
After his falling-out with their father, he lost track of Gwendolen and Cat until a year after their parents' death, when Gwendolen writes him a letter, at Mr Henry Nostrum's prompting, to find out what connection he has with their parents. After going to visit them, he arranges for them to come and live with him at Chrestomanci Castle.
See also Charmed Life (spoilers).