Dennis is a somewhat chubby boy with dark brown/soft black hair. He wears a blue sweater and dark gray pants.
Dennis has a skeptical nature and often questioned Uncle Grandpa's antics. He's also quite easy to irritate and is quick to get impatient (as shown in Uncle Grandpa Ate My Homework when he yelled at Uncle Grandpa several times about eating his homework and picking the buttons in the elevator). However, this irritation and impatience may be a direct result of the situation Uncle Grandpa put him in.
Dennis doesn't seem to be doing too well in school, considering the project he was doing would determine whether or not he would pass his class (also because Mrs. Numty figured his Uncle Grandpa Adventure was an excuse he made up and scolded him for perhaps putting more effort into his work than his excuses). This could be because Dennis procrastinates (working on a school project the night before it's due) or because Dennis does not follow the school's orderly structure (since he is skeptical and impatient by nature as we see throughout the episode).