Marcus Blake is Allison Carter's brother. He drops by to visit his sister's new husband, Jack Carter. Marcus was liked by many people of Eureka because of his book. Marcus told Allison that Jack doesn't match up with Allie's previous husbands. It was only until Kevin gave Jack a drug to improve his memory that Marcus thought Jack wasn't completely dumb. However, he still felt this way when he wasn't surprised Jack had questions about his book. He gained respect for Jack when he helped him move forward with his work.
They later turned ANDY into a super computer and Marcus learned Jack really wasn't all that smart. He thought like the old Jack would when trying to find a rampaging ANDY. When they got him back, Jack and Marcus fixed him. They then turned Jack back to his old self. Marcus agreed to be Jack and Allie's witness for their marriage.