Sidney "Sid" is a fictional character in the Hey Arnold! TV Series.
Sid is generally known to be very superstitious, stubborn, timid, cowardly and especially being very paranoid, having once believed to have killed his school principal with a voodoo doll carved from soap, and at another time believing that Stinky was a vampire. There was even an occurrence in which he was obsessed with cleanliness after learning about germs in his classroom, so much so that he wore a special anti-germ suit and could barely sit inside his desk while wearing it. His paranoid tendencies often cause him much grief.
Sid is Gerald's "lead-in" man; when Gerald is about to tell another of the Urban Legends, Sid introduces him, usually mentioning how the story has been passed "from kid generation to kid generation".
For the most part, Sid hangs around with Harold and Stinky, and the trio is often the cause of trouble around P.S. 118 and the surrounding neighborhood. A few episodes are based around the trio and the consequences they or others must face because of their troublesome antics. Sid can also be a jerk (both when he's with Stinky or by himself) and is rather quick to mock or distrust others. However, he does have his nicer and helpful moments (usually if he's not with Stinky and/or Harold).
Sid's most noticeable feature is his long nose, which seemingly protrudes from his forehead. He wears a green cap in a backward position that covers part of his eyes, a pair of prized white Winklepicker Beatle boots, blue jeans, a white tank top (which was shown in "Eugene, Eugene!"), and a black jacket.
Sometimes, Sid has a "full" head of hair ("Heat", "Save The Tree", "Fishing Trip"), while other times, it's all stringy, like the hairs that stick out from under his hat ("Helga's Makeover", "Wheezin' Ed"), and occasionally (as in "Arnold's Room"), he has both.
Sid's pajamas are usually shown to be green, in "Save the Tree"; however, Sid is seen wearing different pajamas which include a yellow version of his usual hat.
In Hey Arnold!: The Jungle Movie, Sid looks almost exactly the same, with the only difference being he now has a green shirt instead of a white shirt.
- Sid is loosely based on a cousin of Craig Bartlett, whom he thought was hilarious when they were both young.
- In "Arnold's Room", a photo of Rhonda is seen lying on the floor of his room, implying that he has a crush on her. Additionally, they were partners in "Wheezin' Ed", and he jumped in her arms when he was scared.
- Sid takes cooking classes. This had been inspired (although it's likely coincidental) by Sam Gifaldi's father, John Gifaldi, who's a chef and writes cookbooks. 
- He thoroughly enjoys catching frogs, and his pet frog Sidney is actually the first frog he caught.
- Arnold believes Sid has progressively exceptional photography skills, but in "Sid the Vampire Slayer", he messes up a picture of Stinky; a fact which is also commented on by Arnold. 
- It's shown that, like Arnold, Sid has had his signature green hat ever since he was a baby. 
- In the pilot, he's depicted with a bucktooth, his hair also appeared to be ginger (despite the stringy hair still being black), he wore yellow pants, his Beatle boots were completely black and his hat was a pink-like color.
- According to Craig Bartlett, the reason why he always breaks down into crying fits was that his voice actor was such a hilarious "crier".
- Sid is one of several characters to have been voiced by an openly gay actor (his voice actor being Taylor Gifaldi). Other characters include Mr. Simmons (voiced by Dan Butler), Eugene Horowitz (voiced by Blake McIver Ewing) and Kyo Heyerdahl (voiced by George Takei).
- In “Arnold & Lila” Sid has a crush on Lila because Stinky told him and Arnold that when he grows up he's gonna ask Lila to be his gal and Sid responds with "Not if I ask her first." But at school, he, Harold and Stinky tease Arnold about him and Lila being together but it was a cover-up so Harold wouldn't tease him for having a crush on Lila
- Sid's favorite music genres are rock and punk.
- Sid likes to play guitar, especially air guitar.
- Sid's favorite reptile is a frog.
- Sid's favorite colors are black and white. 
- Sid likes horror movies.
- Sid hates vampire movies. 
- Sid has his own bathroom.
- See Sid/Appearances
- See Sid/Gallery