Arnold was created in 1985 by Craig Bartlett, who was also responsible for the clay-animated Penny shorts on CBS's Pee-wee's Playhouse. The original Arnold was visualized as a rich, prep school kid with a vivid imagination, who always wore a prep-school uniform and his signature cap, and was rendered in clay in a series of shorts, one of them televised in 1991 on Sesame Street, and continued to air there, even after the Nick version debuted.
In 1991, the Hey Arnold comic stories, written and drawn by Bartlett (who's also a comic book artist), were published in Simpsons Illustrated magazine (Bartlett is the brother-in-law of Matt Groening, creator of The Simpsons).
The familiar, cel-animated Arnold came about in the mid-1990s when Nick picked up the new series. In the TV series, Arnold is a 4th grader who lives in a boarding house with his paternal grandparents, Phil and Gertrude. Apart from the animation style, Nick's Arnold now wears a sweater, with his plaid shirt untucked (often mistaken for a kilt). Only Arnold's beloved cap remained from the original wardrobe. He no longer wore a school uniform in the TV series because he attends a public school. Now he wears a dark blue jacket to go with his outfit.
Even for a kid, Arnold is notably small in stature compared to most people his age, being shorter than a good deal of his school friends and classmates. His most notable physical quality is his head is shaped like a giant American football (or a rugby ball), thus earning him the nickname "Football Head". In earlier episodes, his head looked longer and more like a blimp, before settling for the current design. He has blonde hair with a tiny blue cap in the middle. His normal attire consists of a teal sweater over a long-length red plaid shirt (which often is confused for a skirt), blue jeans, and black shoes.
In The Jungle Movie, Arnold wears a dark blue jacket on top of his previous outfit, and his shoes are now red and white Converse sneakers. He's also slightly taller, as his plaid shirt no longer looks like a skirt. His outfit during the field trip to San Lorenzo consists of a blue t-shirt with a teal border and sky blue stripe, red plaid shorts, white socks, and brown shoes.
In the pilot, his sweater is orange and his undershirt is blue. He also wears pale red (almost pale pink) pants and light blue shoes.
Arnold is a calm, nice, kind, optimistic, upbeat and smart idealist who always sees the best in people despite how unkind, outlandish or sneaky some of them maybe. Although he's usually a pacific kid, he has his limits and can be a bit irritable when he's pushed too far.
Arnold has a natural gift for giving advice and guidance to others. He often goes out of his way to help others (whether kid or adult), giving them advice when he runs into them, helping them overcome their issues, or help them see the error of their ways. This trait is likely inherited from his parents, who were humanitarian helpers of jungle communities.
Arnold's personality somewhat changed throughout the series. In the earlier episodes, he was a daydreamer and tended to lose focus. And while in earlier seasons he was a regular kid with a moral conscience and a good heart that sometimes made mistakes, at around with Season 4, his character traits of optimism, giving advice, and being the voice of reason became much more pronounced. The episode "Deconstructing Arnold" more or less lampshades this change in Arnold. The last two episodes and the films somewhat revert him back to his original self.
Arnold resides in a boarding house called the Sunset Arms and goes to an elementary school simply named P.S. 118. Arnold's excellent upbringing is due to the way his paternal grandparents and boarding house residents raised him. He's a respectable young gentleman at home and at school with his peers and teachers.
Arnold has often been shown to be dense and blind when it comes to romance and love. For example, he never figured out Helga was "Cecile" in "Arnold's Valentine" (even though it was obviously her), and he doesn't seem to process Helga's incomplete love declarations in "Grudge Match" and "Arnold's E-Files", or question some of her actions involving him. Furthermore, he continually refused to accept that Lila Sawyer didn't return his feelings despite being told so repeatedly. Sometimes, Arnold is shown to be a bit shallow, as he only liked Ruth and Mary Margaret because they were pretty and didn't take time to get to know them.
Arnold's bedroom is full of interesting technology: a remote control that commands everything in his room, a powerful computer and a collapsible couch. His entire ceiling comprises an expansive skylight that includes a trapdoor where people on the roof can come in. Most of his room is blue since it's his favorite color.
Arnold is a natural athlete with impeccable baseball and football skills; he's often playing sports with his friends at Gerald Field.
Though his religion is never explicitly mentioned in the show, it is implied that he belongs to the Roman Catholic faith in a comic titled "Arnold Eats His Dinner". He is involved in a prayer which includes a mention of the Holy Ghost (God). He is also seen attending church in "Arnold Escapes From Church."
Arnold was named after his mother's father. The name Arnold is of German origin, and stands for "the eagle rules" or "strong as an eagle."
Arnold's last name was never properly revealed on the show. Throughout the series' original run, the mystery surrounding Arnold's full name became a running gag, with the last name almost being revealed, then someone or something interrupting or otherwise preventing it from being heard (much like the state Springfield is in on The Simpsons). Some notable occasions when the name is almost revealed include:
- In "Crush on Teacher", Helga does one of her many passionate monologues about her love for Arnold, she says that she would one day like to become "Mrs. Arnold... wait, what is his last name?", revealing that she is just as clueless as to the viewers.
- In a Nickelodeon bumper for the show, Arnold talks to the viewer to straighten out some facts about himself, such as the revelation that his notable red outerwear is a shirt, not a skirt. He ends with, "And yes, I do have a last name. It's..." at which point the screen turns into static and the sound cuts off.
- In the episode "Fighting Families", Arnold wins the school's raffle drawing for the chance to be on the game show Fighting Families. The lady doing the drawing announces, "The winner is Arnold... hmm, there seems to be a smudge over the last name." Here it is revealed that Arnold is the only one in the school with this first name, thus making his surname somewhat unnecessary.
- A similar instance to the one above occurs in "Eugene, Eugene!", where Mr. Leichliter is casting the school kids for an upcoming play, and says, "I can't read my own handwriting" when he's casting Arnold as the villain. However, that scene did reveal Lila's last name (Sawyer, as mentioned above).
- In "Married", Helga dreams that she is married to Arnold and that she is the president. In her opening speech, she introduces Arnold as "Arnold Pataki".
- In "The Journal", Phil is reading the part of Miles's journal that recounts his and Stella's wedding. He gets to the part when they are pronounced "Mr. and Mrs...." at which point Grandma interrupts with a loud belch.
Series creator Craig Bartlett had planned to finally reveal his last name in the second feature film for the series, Hey Arnold! The Jungle Movie. However, the low gross of the first film led to the cancelation of this second film. Years after the cancellation, Bartlett revealed in a chat that Arnold's last name was also a nickname, and Grandpa was the character that said it the most throughout the series. As his Grandpa's nickname for him is "short man", many were led to believe that his last name was "Shortman".
- At the beginning of "Arnold Visits Arnie", when Helga is telling the other characters their positions in a game of baseball, she says "Looks like you're on shortstop, Shortman." This was the only time his last name was directly said in the original series.
- In "The Journal", a toy bus can be seen right next to baby Arnold, that has his last name on the front of the toy.
- "Shortman" is an actual surname. Its origin is English, with the earliest known variant of the surname recorded in Dorset in 1176. 
- In 2016, after production on The Jungle Movie restarted, Craig revealed in an interview with BuzzFeed that Arnold's last and middle name are indeed Phillip (his middle name) and Shortman. This was cemented when the film aired the following year, with a scene of Arnold signing his last name.
Arnold was born in San Lorenzo, in a temple of the mysterious Green-Eyed People during a volcanic eruption. His birth "silenced all of nature" and the volcano, leading the Green Eyes to believe Arnold would one day become a messiah of sorts to them.
As stated in "Married", he was born on the 7th day of an unknown month. Most fans agree that he was born on October 7th, the day the first episode of the series aired for the first time. Additionally, in a chat, Craig Bartlett said, "...Arnold is like me in that he has to think things over a long time, before coming to a decision. That I believe is a Libra characteristic. Go Libras." This implies Arnold being a Libra, which would make his birthday fall sometime between September 24th to October 23rd, essentially confirmed he was born on October 7th.
Arnold never dates anyone, but he does have crushes on girls. Throughout the series, his relationships are one-sided on his side.
CrushesArnold has had two major crushes in the series: 6th-grader Ruth P. McDougal (during Season 1), and a classmate named Lila Sawyer (who was introduced in Season 2). His crush with Ruth ended in "Arnold's Valentine" after he went on a date with her and realized she wasn't his type. His crush on Lila ended when he became embarrassed after she overheard him tell his best friend's younger sister, Timberly, to help convince Lila to reciprocate his feelings in "Timberly Loves Arnold". In the episode "Crush on Teacher" Arnold develops a crush on his substitute teacher Miss Felter and even went as far as to think she had a crush on him too. The crush on her ended at the end of the episode when Arnold found out that Miss Felter didn't actually have a crush on him. In "Summer Love", Arnold develops a crush on a girl called Summer, before Helga exposes that she's just trying to use him.
See Helga and Arnold.
Arnold’s warm-heart and respectable and wise personality earns him friendships among many, if not most, of his classmates and school peers. His best friend is Gerald Johanssen, whose relationship dates as far back as pre-school. The pair have a distinct handshake consisting of putting two-thumbs-up fists together and wiggling the thumbs back and forth.
Over the course of the series, Arnold has had a handful of voice actors. In the pilot, Arnold's voice was provided by J.D. Daniels, though when the series came to fruition, Daniels was not brought on. During Season 1 and "What's Opera, Arnold?" Toran Caudell provided the voice of Arnold, though he left the show after the first season due to his voice changing, returning soon after as Wolfgang. Phillip Van Dyke was brought on as the voice of Arnold for the second and third seasons. However, like Caudell, Van Dyke's voice also underwent changes, causing another change in voice actors (though Van Dyke would return during Season 5 to voice Ludwig in "New Bully on the Block"). In Season 4, Spencer Klein started voicing Arnold and continued to provide his voice all the way through Season 5 and the movie. The last two episodes "April Fool's Day" and "The Journal" feature Alex D. Linz as the voice of Arnold instead. His most recent actor at present is Mason Vale Cotton, voicing him in Hey Arnold! The Jungle Movie.