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).