The Protector of Hillwood

Arnold is a calm 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 centre to those around him, whether he is around his (non-biological) family in his paternal grandparents' 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.

Helga Geraldine Pataki is nine years old and in fourth grade. She is secretly in love with her classmate Arnold because he's the first person to give her attention and be nice to her. In one episode, Helga revealed to Lila that she loved Arnold so that she could get the role of Juliet in the school play and kiss Arnold. Later in the series, Helga admits to being in love with Arnold to the school psychologist, Dr. Bliss. However, it is implied that her best friend Phoebe is aware or suspects that Helga loves Arnold as in when she uses the code word "ice cream" during conversations. During her spare time she likes to devote her time to writing poems (mostly about Arnold) and collecting Arnold's used gum so that she can add it to her shrine of him which is a work in progress. Gerald Johanssen, Arnold's best friend, who is African American. He is the keeper of several of the town's urban legends. Throughout the series both he and Phoebe, another classmate, have a love interest in one another. He is also known for his funny looking afro.

Harold Berman, school bully and occasional friend of Arnold, overweight, revealed to be Jewish. Despite his teasing and bullyish tendencies, Harold is often revealed to be rather cowardly and usually cries for his mother whenever he gets scared. He is sometimes referred to by the derisive nickname "fat boy." Harold is 13 and has been left back in the fourth grade several times. He and Big Patty are shown to have feelings toward each other.

Phoebe Heyerdahl, the smartest girl in the class. She is Helga's confidant. Her ethnicity is half-Japanese. Throughout the series both she and Gerald, another classmate, have a love interest in one another.

Rhonda Wellington Lloyd, a rich yuppie girl, pompous, self-proclaimed fashion queen. She is one of the wealthiest children at P.S. 118. She is also Curly's crush

Lila Sawyer, a nice girl who came from a farming community she calls "Pleasentville" and lives with her father. Nothing is ever mentioned about her mother. She is Arnold's crush. However, she only prefers Arnold as a friend.

Sidney "Sid", a boy with paranoid tendencies. He is always shown wearing a baseball cap backwards and "Go-go" boots.

Stinky Peterson, tall and lanky boy from Arkansas, has a southern accent and a love for lemon pudding. He was originally portrayed in the first season as a bully and sidekick of Harold's, though this was later de-emphasized. He is named so because he often defecates himself and must wear a diaper.

Eugene Horowitz, a geeky, nerdy, optimistic boy, the class jinx. He has extremely bad luck and is often bullied.

Thaddeus "Curly" Gammelthorpe, the class psychopath. He has had outbursts throughout the show such as in the episodes "False Alarm" and "Curly Snaps." He also appears to have a major crush on Rhonda. In the episode "Curly's Girl," Rhonda pretends to be Curly's boyfriend for a week, but is disgusted by his appeal to her.