Mark is introverted and nerdy, sometimes described to be the exact opposite of Bobby and Courtney. He is shown to be very smart and resourceful. He enjoys eating carrots and watching cartoons.
Mark has black curly hair, glasses, and lots of acne.
Mark isn't very close to Courtney when they first meet. She doesn't really know he exists until Bobby leaves and he asks Courtney to help find him. The become close friends while keeping Bobby's journals.
Bobby and Mark have known each other since they were little kids. Though they don't seem to have a lot in common, they make each other laugh and they have fun together. They grow apart slightly over the series, but they still consider one another to be their best friend.
Andy has been bullying Mark for as long as he can remember. Mark hates everything about him, until he joins a science club at school. He's suddenly very, very smart. They work on a project together, called Forge, and become closer in the process. They become even better friends after saving Courtney's life together, and then running away to First Earth. Mark is crushed when he realizes Andy is Saint Dane, and that he's been tricked the whole time.