The eldest son and living child of Anne and Gilbert Blythe, James Matthew Blythe - Jem for short - is the only one out of all his brothers and sisters to have been born anywhere but Ingleside - the House of Dreams. He is named for Captain Jim Boyd, an elderly, retired sailor who keeps the Four Winds light and befriends Gilbert and Anne when they move to the House of Dreams - he dies soon after Jem is born. Jem's middle name is after Matthew Cuthbert, his adopted "grandfather", who dies at the end of Anne of Green Gables. Or, as Anne puts it,  "the two finest gentleman [she] knows".

Jem has curly red hair, frank hazel eyes, his mother's nose and his father's mouth. Jem is also the only one with ears nice enough to please Susan. He is sturdy and reliable, not a great talker but a good all-round student. Jem likes to investigate things through, and constantly experiments and observes. This leads him to know a lot about nature, people and the little world the children live in. He is a "chieftain" at school.

At the start of Rainbow Valley, Jem is thirteen, but still not too old to play with Walter, Nan, Di and the Merediths in Rainbow Valley, a beautiful hollow behind the grounds of Ingleside. He brings about the "Good-Conduct Club", a scheme the Manse children have to bring themselves up properly. Near the end of the book, though, he is fifteen and studying for the entrance to Queen's, and tends to spend more time in the attic studying than down in Rainbow Valley with the others.

Jem is mostly friends with Jerry Meredith and his younger brother Walter, but in Rilla of Ingleside, he seems to be trysting about with Jerry's sister Faith a lot, and the couple are even suspected to be engaged. At the start of Rilla of Ingleside, in 1914, it mentions that he is twenty-one, so that indicates he was born about 1893. Straightaway, after the Great War breaks out, Jem and Jerry enlist to fight, leaving for Valcartier soon after. It mentions that Jem had just completed his first year in medical training - he is going to be a doctor like his father - before he goes.