Arnold is a fictional character in the TV cartoon Hey Arnold. Arnold is dreamer and an idealist who always tries to see the best in people and to do the right thing . Whenever he sees someone in trouble, Arnold goes out of his way to help them out, even if it is not sensible to do so (example, when he tried to protect some of his classmates, who had mooned their principal). Arnold often acts as the stable center to those around him , whether he is around his "family" in his grandparents' (Dan Castellaneta and Tress MacNeille) boarding house The Sunset Arms, or around his friends at his school, P.S. 118. According to the Parents Day and The Journal episodes, Arnold's parents were doing missions in the Central American jungle when they disappeared.

He's had two major crushes in the series: with 5th grader Ruth McDougal, and with a classmate named Lila. Both of these ended after he hung out with other girls and learned about how not-so-great they were (French exchange student Cecile in Arnold's Valentine, then his friend Gerald's younger sister Timberly in Timberly Loves Arnold). Most recently, he's had to come to terms with classmate Helga's confession in the movie based on the series.

Unlike the other major characters on the show (except for Sid), Arnold's surname (and presumbably that of his grandparents in whose boarding house he lives) has never been disclosed in any episode script or other publicly available documentation. There had been plans to reveal it in the series' 2nd film, which would also deal with his search to find his parents, however, problems between the series creator Craig Bartlett and Nickelodeon , as well as the low gross of the first film, have left the future of this 2nd film in doubt.

Throughout the series' run, he was voiced by at least 4 different voice actors: Toran Caudell, Phillip Van Dyke, Spencer Klein (who did his voice in the movie), and Alex D. Linz (who did his voice in the final 2 eps of the series).