Duncan is a tall, fifteen-year-old boy with a red, wild, unkempt hair and pale skin. He wears a green sweatshirt over a yellow shirt, blue pants, and tan sneakers.
He has bad posture which he initially says in Pilot will cause him problems later in life. In Red Head Redemption his sister calls it a "pre-scoliosis hunch". In "Undacuva Mutha" it is mentioned that Annie bought him a back brace, but he hates wearing it as it makes his back uncomfortable
Duncan is easily influenced by his hormones and has a tendency to make bad decisions. He heavily relies on other people like his sister Kimberly in "Red Head Redemption" as he needed her to play his game for him while he was away.
He is apparently not too well known in school, as Classless President reveals many students wrongly think his name is Steve.
He has a crush on Mia Abara.
- He is the Harris counterpart of:
- Bart Simpson (The Simpsons)
- Chris Griffin (Family Guy)
- Steve Smith (American Dad)
- Cleveland Brown, Jr. (The Cleveland Show)
- Gene Belcher (Bob's Burgers)
- Sanford Buckwald (Bordertown)