Mr. Sweeney is usually seen with a lab coat and glasses. He has a round face and pants. Sometimes it looks like he has rosacea.
He is shown to be hard on Ned and Cookie because of their laziness and crazy schemes. He knows they're capable of excelling themselves and sometimes, he offers his assistance to them, especially when he knows one of them is having problems. For example, he was willing to tutor Ned to prepare him for an upcoming exam and felt concern for Ned when he was depressed about Suzie moving away. He is often shown as the show's antagonist. Sometimes, he is just teaching Ned, Cookie, and Moze a lesson. He has a dislike for cell phones as shown in the episode "Cell Phones". He is always shown with a lab coat except for the Episode "School Car Wash." At the end of the series finale Guide to: Field Trips, Permission Slips, Signs, and Weasels, he says to Ned "I'll always remember you as one of my worst students, but also one of my favorites."