Duncan is the younger brother of Shawna, who dated Jack. They met when Jack became his mentor (a punishment Jack received after confessing to making fake IDs for his fellow students at school). Duncan lives with his sister and is afraid he might lose her someday. He is therefore very protective of her.
Duncan tried to pull a con on Jack by pretending to sell candy bar for a made up charity and then stealing the money. Jack retaliated by signing Duncan up to work with a real charity that sold candy bar. Duncan accepted his sister going out with Jack but the relationship didn't last long. After Jack and Shawna broke up they agreed to remain friends and he continued to mentor Duncan.