Harold is immature, naive, cowardly, dumb, insecure, and usually kind of obnoxious, but becomes caring and more innocent after the first season (though he denies it in public). Harold can be considered the bully of the group, even though his bark is usually worse than his bite. Despite his frequent posturing, he is also a bit of a crybaby and is easily frightened and/or upset and frequently cries for his mommy (among other things) while running away or wanting to escape from a situation.
Harold can be somewhat of a backstabber: One day, he can be a friend/ally, but then the very next day, he's teasing and bullying. He is usually seen with Stinky and Sid, who both share this character trait as well, usually causing problems for others or themselves.
Harold is overweight and is known to eat a lot, as exhibited in "Weighing Harold," in which he puts on an enormous amount of weight. He wears a blue baseball cap, a light blue short-sleeved jacket with dark blue trimmings over a white shirt, which reveals his belly and wears grey shorts with black shoes. He also has yellow eyes (which are sometimes white) and has a large bottom tooth sticking out of his mouth.
Harold's facial features resemble his mother's, though he also has a unibrow like his father.
As of Hey Arnold! The Jungle Movie, Harold hasn't really changed in appearance, though he appears to have slightly grown and gained more weight, as his clothes now seem to fit tighter.
In "Harold the Butcher", it is revealed that Harold has a deep love for meat products. Harold's distinctive features are his fat stomach and his giant misshapen tooth. In some episodes, Harold has yellow eyes, although they're white in the title sequence.
Harold often gloats of his own intellectual or physical superiority, though usually seconds later, he proves himself wrong or is shown up by another character. Harold also has a budding relationship with Patty Smith. He is of the Jewish faith, as revealed when he invited the other students/friends to his Bar Mitzvah (his Hebrew name is Chaim).
Harold may also not be as unintelligent as he leads himself to believe. In "The Aptitude Test", an accidental mix-up in test results suggests he may be a genius (rather than Helga, who got his pathetic test scores in exchange), resulting in him striving to remain true to this fact. However, neither before, nor after the events of this episode, does Harold show any outstanding intellectual skills.
Harold's uncle owns an alligator farm (as revealed in "Return of the Sewer King").
It's implied that Harold may have a crush on 6th grader, Patty Smith. They became close friends in the episode "Hey Harold!" and it continued up to the episode "April Fool's Day". However, at one point, they become rivals in an arm wrestling contest in the episode, "Harold vs. Patty". These episodes also show his true potential and a different and softer side of him.
It's also implied that Harold could be romantically interested in Rhonda. In the episode "Operation Ruthless", Rhonda is seen pulling Harold to the Love Tunnel's boat while saying, "I know you like me, Harold!" as he tried to resist. Then, in "Love And Cheese", for a brief second, when Arnold and Lila walk to the Tunnel of Love, you can see Harold and Rhonda sitting together in the swan boat.
In "Egg Story", Rhonda and Harold are assigned as parents to take care of an egg. Harold reminds her of their moment in Operation Ruthless (Or possibly Love and Cheese), but Rhonda tries to keep him quiet about it. The egg experience ended with Harold frying the egg and trying to fix the shell. At first Rhonda got very mad, but then forgave Harold when she realized how hard he tried to put the pieces back together, and the two shared a meaningful look.
For the most part, Harold tends to pal around with Stinky and Sid. The trio often cause trouble, which always results in either them or another character dealing with consequences of their actions. Because of this, several episodes are based around the trio's antics, most notably in "Full Moon", when the trio decide to moon Principal Wartz, only to have Arnold almost take full responsibility for their prank. However on a few occasions Sid and Stinky make fun of Harold (his weight and losing to and being in love with Big Patty). Ironicially, Stinky played the role of sidekick to Harold during his initial portrayal in the first season of the series as a local bully.
- In some episodes, Harold's outer shirt has a red "H" on it, but in other episodes, it's just a plain blue shirt.
- Usually, when Harold apologizes in the later parts of Hey Arnold!, the song "Vesti la giubba" from Pagliacci plays. Possibly because the music was used for his song "Big Ugly Clown-o!" in "What's Opera, Arnold?".
- His voice actor Justin Shenkarow also voiced Eddie from Lloyd in Space and several minor characters on Recess. Lloyd in Space and Recess are both shows that some of the cast members (such as Toran Caudell, Francesca Marie Smith, Sam Gifaldi and Blake McIver Ewing) have been in.
- He is similar to Binky Barnes, a character from Arthur. They both started as bullies at the start of the series and later, as friends of the hero. They are held back at school. Also, Binky and Harold look similar.
- Harold loves meat (as seen in "Harold the Butcher"). Upon that episode, Harold got to be Mr. Green's apprentice. It is implied that Harold may regularly work at the butcher shop on Mondays after school.
- In the pilot, claymation shorts and original comics, Harold wore a red striped shirt and a crown that strongly resembled the one that Jughead from Archie Comics always wore. In the pilot he wore his hat, however it was shown as a turquoise-like color.
- He once cared for a stray kitten he named Cupcake after finding it eating out of garbage cans in the episode "Harold's Kitty". Which shows his light-hearted side.
- In the Baseball Uniforms comic, he is seen wearing briefs as his underwear, however in the show he wears boxer shorts.
- In the episode “Polishing Rhonda” Patty has a picture of Harold in a pink heart shaped picture frame on her vanity table in her bedroom just like Helga G. Pataki having a picture of Arnold Shortman in her gold heart shaped locket.
The episode "Harold's Bar Mitzvah" established that Harold is 13 years old, and "Hey Harold!" followed up on this by clarifying that he's in the 4th grade because he's been held back a few years (which isn't surprising, given his academic performance in other episodes). However, "Helga on the Couch" and "The Journal" show Harold in preschool and as a baby, respectively, along with the Arnold and the rest of the class with no signs of any age difference, despite him supposedly being 3-4 years older than them. Not to mention that despite being 13, Harold is nowhere near the size of Torvald (who is the same age), or even Wolfgang (who is 2 years younger, unless he's been held back as well).