Gerald Martin Johanssen is Arnold's best friend and the deuteragonist from the television series and first film of Hey Arnold!, and also was the tritagonist of the 2017 TV film Hey Arnold!: The Jungle Movie. He is almost always on the scene when Arnold's latest escapade happens. He is the occasional voice of reason to Arnold and also has a good deal of knowledge of many legendary stories in the city, or "urban legends" as he calls them.
At school, Gerald is the class president, and considers himself very cool, and apparently, so do his friends. When it was revealed that Arnold was on Rhonda's so-called "Cool List", and that Gerald was on her other list known as "The Geek List" (due to a grudge Rhonda had against him over running for class president), many classmates were surprised to find this out. He can even become jealous of others if they steal his spotlight, which likely stems from his home life, where he is the oft picked on middle child.
He often serves as the voice of reason in the group of fourth grade friends, and a realistic counterbalance to Arnold's optimistic attitude. This sometimes causes conflict between the two, but never for long. Another facet of Gerald's personality that opposes Arnold's is his weaker sense of morality. Gerald is far less open to helping others, and almost always attempts to dissuade Arnold from doing so. He's also more likely to doubt people, or to spite somebody that he believes has wronged him, such as his brother. He's also somewhat selfish, seen when he hesitated to tell Jamie O that his girlfriend was manipulating him.
Gerald is of African-American origin, and his most distinct features are his hi-top fade (classmate Helga often calls him "Tall Hair Boy"), and his red shirt with the number 33 on it. As a child, his red shirt simply said "3". He also wears blue jeans and red and white high top sneakers.
In The Jungle Movie, he wears a red hooded sweatshirt with the same number on it, and different high top sneakers of the same color with white laces. He resembles his brother Jamie O more closely, as his haircut is now a taller version of Jamie O's, and the shape of his head is now less circular, and more similar to his brother's.
He serves the role of storyteller in the fourth grade group, and his tales are often the setup of the urban legend episodes. He sometimes delivers the urban legends in the form of jazz poetry, which he also recited on stage at the Coco Hut. Gerald's flair for performance has also manifested musically, such as in the episode "Gerald's Tonsils", Gerald is a soloist of chorus, in "Partners", he plays the piano, and in the episode "Arnold as Cupid" he's briefly seen playing violin.
He notably possesses a lot of information and know-how on Hillwood and its residents, some of which provided by his mysterious friend Fuzzy Slippers.
He developed a fear of riding a bike after he accidentally rode down the steepest hill in the city, and crashed, when he was small. However, Arnold helped him over come his phobia, and win the city bike race.