Arthur is Jeremy's pet pig. He won him in a pig lotto. Jeremy won Arthur. Not the other way around.
Nilson "Nils" Kerr is Jeremy's best friend, "constant companion" and apparent love-interest, or maybe something more complicated and subtle. But in any case they hang out a lot. She's fond of hitting things.
Jack Leaderboard is a freelance private-eye who does a spot of pro-bono work trying to discover who iced Dougal. He also writes crap semi-autobiographical fiction, possibly as a tribute to American underground comics.
Angus MacConnor is Jeremy's uncle and some species of scientist. He isn't married to Jeremy's Aunt Lotte, so I'm guessing brother-and-sister. He vanished mysteriously.
Jeremy Lachlan MacConnor is a physicist. He invented a dimension-hopping thing called a "skip," which looks remarkably like a winnibego with jet engines gaff-taped to the sides. With this he, Nils, and Zeigfried ran "D-Tours", the Interdimensional tours company. They also worked together at the Platinum Grit cafe.
Lotte MacConnor is Jeremy's aunt. Nils once described her as, "A dribbling tartan frothbucket." Lotte raised Jeremy at Castle MacConnor in Loch Snuff, Scotland.
Dougal MacWikkening is Jeremy's bully of a cousin. He's "mostly immortal," along the lines of Highlander. Nils dropped a piano on his head, and now he's presumed deceased.
Kate Provoczki is a journalist and former flat-mate of Nilson's. She and Jeremy once drove from Scotland through Denmark, Tijuana, the North American midwest and back, without even crossing the ocean. Magic, I'm guessing.
Raoul the Talking Wardrobe has a Jamaican accent.
Zeigfried owns the Platinum Grit cafe. He is excruciatingly German.