Arnold is nine years old throughout the run of the TV series. If we were to figure out a year of birth for him, we would need to use Helga's birthday as a guide. During an interview with Arun Mehta, Craig Bartlett had quickly figured that Helga would have been 9 in 1996, the year "Hey Arnold!" had first aired on television. Throughout most of the series, Helga had expressed that she was 9, but in "April Fool's Day", which took place a week after her birthday in 1997, Helga would have been 10. Since Arnold and Helga overlapped at nine years of age in the series before the Spring of 1997, Arnold would not yet have celebrated his tenth birthday. Since fans have figured out that Arnold's Birthday may very well be on October 7th, it can be deduced that his D.O.B. is October 7th, 1987, making him about 5.75 months younger than Helga (assuming that's his actual birthday).
Arnold's favorite genre of music appears to be Jazz. In various episodes, he expressed interest in the music of Dino Spumoni and in the episode, "Timberly Loves Arnold", he suggested to Gerald that they go look through Thelonious Monk records at a record store. His love for Jazz may be due to the exposure that he had to the genre while growing up in the Boarding House with his Jazz-loving grandparents. Not only does Arnold appear to be a fan of Jazz, but he also seems to enjoy Funk, as he had danced to a "James Brown-esque" song in "24 Hours to Live". Arnold also appears to enjoy Disco music, as he and Gerald danced to it in the episode, "6th Grade Girls". He also shows an appreciation of Country Music, as shown in "Mr. Hyunh Goes Country". Another genre that Arnold may be into is Rock (and he may like it hard and loud). We see this in the episode, "Arnold's Room". When Sid first enters his room and turns on the stereo, the radio or CD changer is set to play Heavy Metal music, and it is quite loud (very similar to Helga's taste in music). For the most part, Arnold's overall taste in music appears to be pretty open and eclectic.
Arnold is the only character to appear in every episode of the series, although some of them, he had only a minor appearance in.
Arnold appears to be gifted with artistic talent, as mentioned in "Part time Friends" and "Rich Guy".
According to "The Aptitude Test", Arnold would make an excellent ambassador to a foreign land.
Arnold is capable of perfectly mimicking Grandpa Phil's voice as shown in "Hooky" because he has spent so much time around his grandfather.
Any girl Arnold has had a crush on throughout the series has either not returned his affection or has turned out to be a jerk.
Arnold has an emotional attachment to his hat as shown in "Arnold's Hat". This is because it was a gift from his parents, as seen in "The Journal". Yet, he has never cried once.
He and Gerald have been friends since at least preschool, as shown in "Helga on the Couch".
Arnold may be subconsciously in love with Helga as seen in episodes such as "Arnold's Valentine", "Arnold Visits Arnie", and "Married".
Arnold loves watching sports and playing them, but appears to favor baseball, as shown in "The Baseball", "Dangerous Lumber", and "Rich Guy". He appears to specialize in baseball but also appears to be good at football, especially with coming up with plays as shown in "Mud Bowl".
Arnold appears to be the stable center of the Sunset Arms, as he helps to stabilize the occasionally hostile relations between the boarders.
Evidence of this is shown in "Gerald Comes Over", wherein Arnold collects the rent and breaks up the fight between the boarders.
Arnold's room is accessible via ladder and appears to be the biggest room in the boarding house and is a part of the attic with its own panorama view.
According to "Married", his favorite color is blue and he was born on the seventh of the month, which is also the number on his jersey in "Benchwarmer"
Arnold at first glance appears to wear a kilt but in actuality it is his shirt that is left untucked.
Arnold's compassion extends to animals, as shown in "Field Trip" and "Big Caesar".
Arnold has won an eating contest in "Eating Contest".
Arnold is shown in multiple episodes, to have a taste for tapioca pudding, which Helga shares.
Arnold has a foldable couch in his room.
Arnold has an alarm clock that looks like him and wakes him up by saying "Hey Arnold" consecutively as it clasps the upper half of its head up and down like the cymbals of a drum set.
Arnold is close to his pet pig, Abner and when Abner went missing in "Abner Come Home" he searched throughout the entire town to look for him.
It is shown in "Friday the 13th" that Arnold is not superstitous.
There have been a number of episodes in which Arnold does something that brings misfortune to Eugene, such as "Eugene's Bike", "Eugene's Pet", and "Eugene's Birthday" and when Arnold tried to make it up to Eugene, his efforts went terribly wrong. However, he did prove to also bring him good luck, as he helped prove his innocence when he was accused of pulling the fire alarm in "False Alarm", when he resuced Eugene from getting mugged in "Mugged", and cooperated with him to fix the play in the episode "Eugene, Eugene!" which Leichliter changed drastically.
Despite the two not having an "official" relationship throughout the series, Helga and Arnold have kissed multiple times. All of these kisses were instigated by Helga and they occurred in "School Play", "Summer Love", and Hey Arnold: The Movie. They also kissed multiple times in "Married" although all the kisses happened in their dreams.
Arnold seems to often come across something that would expose Helga's feelings for him and Helga always has to sneak into the boarding house to prevent him from finding out. These episodes include "The Little Pink Book", "Helga's Locket", "Helga Blabs It All", and "Helga's Parrot".
Helga has attempted to confess to Arnold in "Helga Sleepwalks", "Helga Blabs It All" and "Monkey Business" but was interrupted each time.
Arnold became aware of Helga's feelings in Hey Arnold: The Movie but agreed with Helga that it was just part of the heat of the moment. However, he seemed to be aware it was not part of the heat of the moment. This appears to be true as he flirted with her during their tango in "April Fool's Day", which takes place after the movie.
Whenever Arnold has attempted to "get back" at Helga for something she did, it never works, as shown in "Girl Trouble" and "April Fool's Day."
Arnold appears to be a good tango dancer, as shown in "April Fool's Day."
In "Helga's Parrot", Helga apparently saw Arnold naked as Arnold was getting undressed and Helga was hiding behind his foldable couch. She closed her eyes for a few seconds before opening them up again and fainted while smiling.