Arnold is a fictional character and the titular protagonist in the Nickelodeon animated television series Hey Arnold!. His head is shaped like an American football, thus earning him the nickname "Football-Head", by Helga. Throughout the series' run, he was voiced by at least five different voice actors: J.D. Daniels (who did his voice in the pilot), Toran Caudell, Phillip Van Dyke, Spencer Klein (who did his voice in the movie), and Alex D. Linz.
Arnold is a 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 (an example being when he tried to protect some of his classmates, who had mooned the school 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. He is picked on, especially by Helga, for his trademark football shaped head. He has spikey blonde hair that sticks up and a tiny blue baseball cap that sits on one side of his head. He also appears to wear a red and yellow plaid "skirt" over jeans, though it is later revealed to be the bottom of an undershirt poking out from his sweater. Helga has once compared his eyes to green jellybeans in one of her many love poems about him.
According to the Parents Day and The Journal episodes, Arnold's adventurous parents, anthropologist Miles and biologist/physician Stella, were doing missions in the jungle of San Lorenzo when they disappeared. According to those episodes, Arnold was born while his parents were on an earlier expedition in the jungle to bring medicine to the natives. His birth coincided with the eruption of a nearby volcano, which the locals believed to be an indication that Arnold would one day become a messiah of sorts who would save the world from a great danger. Arnold was not told of these events until later in the series when it was revealed by his grandfather. It is unknown how the "prophecy" may have fit into the series had it continued.
He has a love for music, particularly jazz. He is seen in one episode discussing with his friends various sound systems, owning himself, an advanced sound system. He also taught himself to play harmonica and has played it a few times throughout Seasons 1 and 2. His room contains interesting technology such as a remote control that controls everything in his room and a collapsible couch. His entire ceiling comprises several skylights as well.
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Official Site: Nickelodeon
Links: Arnold's Wiki Article Hey Arnold Fansite TV.com