Marcus is Jesse's best friend since they met in fourth grade, after the Fairn family moved to Seattle. He has a younger brother Eric, whom he teases mercilessly. He is described as "sturdily built" and muscly, and Jesse thinks he's a "dork", intelligent but not academic. By 2016 he is training in the hope of becoming a firefighter.
Jesse spent much of the summer he turned 14 at Marcus's house (while Indio was in Highvale and Caleb was deployed). Jesse's tradition of giving Christmas stockings to the family came about because he spent the Christmas he was 15 at Marcus's house, and Marcus's mother made him a stocking filled with gifts.
After high school, Marcus went to work in his father's hardware store instead of going to college.
In April 2014, Marcus brings his girlfriend Paislee (Paiz) to his cousin Vivienne Benedict's Smoke and Mirrors costume party (04 Lost Melodies), which is Marcus's first appearance in the series although he is mentioned in earlier books.
He and Jesse attempt to set up Eric and Wynter. Marcus bets Jesse $20 that he'll never ask her out. Marcus tells Eric why Wynter has scars, which causes a rift between him and Jesse that lasts eighteen months. They reconnect at Christmas 2015 (09 Broken Strings) and plan a trip to Mexico in the summer of 2016.