Voiced by: J.D. Daniels (original pilot); Toran Caudell (season 1 and "What's Opera, Arnold?"); Phillip Van Dyke (seasons 2 and 3); Spencer Klein (seasons 4, 5, and The Movie); Rusty Flood ("Helga on the Couch" and "Parents Day"); Alex D. Linz ("The Journal" and "April Fool's Day")
The Ace: He can do anything. By the time of the movie, most of the neighbourhood believes he can save them from having to sell up and move. And he does.
Et Tu, Iggy?: Happens to him in the episode, "Arnold Betrays Iggy", when he was forced by him to wear his bunny pajamas. This is the reason why many fans hate that episode. even Craig Bartlett and the producers hate it. So they made it like a "lost episode"
Announcer: And the winner is...Arnold...hm. The last name appears to be smudged.
Stinky: That must be you, Arnold, on account of you're the only Arnold in the entire school.
Only Sane Man: Amongst his class mates, though Gerald isn't that far behind.
Parental Abandonment: Living with his grandparents, Arnold's Disappeared Dad and Missing Mom are not even alluded to for a good portion of the series. We eventually learn they disappeared during a jungle expedition while Arnold was a baby.
Breakout Character: Helga grew in popularity due to her compelling Character Development. By the end of the series, most people would think she was the main character instead of the football head due to how much spotlight she had. In fact, the show was meant to have a sequel specifically geared around Helga as the main focus in high school.
Brilliant, but Lazy: Though not a perfectionist like Olga, the aptitude test shows she is nearly as competent.
Broken Bird: Of the badass variety. She mentions in one episode that she would probably go crazy if it weren't for Arnold.
Drama Queen: Even taking into account her usual hamminess and issues, she's usually the first person to crack up and go melodramatic in a crisis. (Ironic, considering she's considered the toughest girl in school behind Big Patty.)
(when stuck on the other side of town) "Walk!? It'll take days! We'll freeze to death! WE'LL GET EATEN ALIVE BY PIGEONS! Or worse... rats!"''
Even Evil Has Standards: Does some nasty things to Arnold, but takes major offense to her father calling Arnold an orphan within ear shot of him. In "Curly's Girl", Helga calls out on Rhonda for embarrassing Curly after dumping while pretending to be Curly's girlfriend.
Helga: I think Miss Rich and Fancy did a number on him, (referencing to Curly).
Rhonda: May I borrow the black paint?
Helga: (paints a heart with the black paint), perfect, it'll match the color of your heart.
Rhonda: (annoyed), excuse me!
Helga: You heard me Cruella.
Evil Laugh: Has a very impressive one. Not as psychotic as Curly's (but then, who could be), but she gets points for once doing it in a theatre, with ominous music and everything.
Evil Plan: She explicitly refers to them as such, and she has a ton of them. They nearly always fail in ways that harm her and help those she schemes again.
Large Ham: A running gag is her tendency to break into overly melodramatic monologues whenever she does anything she regrets, as well as villainous monologues when she's scheming. Often complete with theatrical music.
Like Father, Like Daughter: Personality-wise, she actually has a lot in common with Big Bob, but Helga is much more articulate and emotional.
Unknown Rival: Lila has no idea that Helga hates her at all, let alone why. Ironically she likely wouldn't want to be considered Helga's rival for Arnold's affections at all, since she doesn't reciprocate.
Berserk Button: She seems to have a mild one connected to when someone outperforms her academically or socially. In "Phoebe Cheats," she plagiarizes a poem for a contest presumably because Sheena got a higher grade than she did, and because she wasn't chosen as a Student of the Month.
Helga: "Oh, and Phoebe? This conversation never happened."
Characterization Marches On: In the first season she was a lot more of a Satellite Character to Helga, to the point of more or less being her shadow and not having much character herself. Later seasons fleshed out her personality more and made her less of a "minion" type.
Sidekick: Though she hates being treated like one, she often fills this role. As her character is fleshed out more it's shown more clearly that it is due to her being an extremely loyal - and patient - friend.
One of the most popular kids in school and self-proclaimed "fashion queen", Rhonda is spoiled, rich, obsessed with fashion, and is a bit of a snob, but she does have a friendly side that comes out quite often. Her best friend is Nadine, but she's often seen hanging out with Arnold and the gang. It's implied that she and Harold like each other, and Curly likes her too... much to her chagrin. She's also a surprisingly good football player.
Heroic BSOD: When her family goes poor in "Rhonda Goes Broke" - specifically, when everybody finds out that she's poor - Rhonda has a major one, spending all her time in her room miserably crying. Luckily, Arnold reboots her when he talks some sense into her and asks if being rich is all she's about.
Hypocritical Humour: In one episode, she tells Nadine she has to have a "three shoe rotation" in order to continue hanging out with her, even though all the characters in the show, including Rhonda, have a Limited Wardrobe.
Held Back In School: Is about 13 years old (old enough to have a bar mitzvah) but still in 4th grade.
Hidden Depths: Usually with Harold what you see is what you get. But after a mix up in "The Aptitude Test" results in him believing he's a genius, he starts listening to classical music, correctly solves a math problem in his head, and designs a gazebo/bonsai garden for his family's rooftop.
Informed Judaism: His Jewish background wasn't mentioned until the episode in which he tries to run away from his own bar mitzvah, though prior to that, it was established he had a mom who acted like a Jewish Mother.
In fact, in one of the old claymation shorts, he attends church with Arnold.
Ship Tease: With Patty, and Rhonda in some episode.
Vague Age: The writers seem to flip-flop between having Harold a few years older, or the same age as Arnold and his friends. In one episode he is shown to have gone to preschool with them yet in another he turns 13.
Voiced by: Sam Gifaldi; Taylor Gifaldi ("April Fool's Day")
Only One Name: The second most prominent example on the show, behind only Arnold himself.
Pet the Dog: He's not all bad, however. In one episode, he gets into trouble with Big Gino. When Arnold tells Sid he doesn't have to do what Big Gino says, Gino orders him to give Arnold a swirlie. Sid stands up to Gino to take Arnold's place.
With Friends Like These...: Shares this status with Stinky as two of the worst of the kids besides Helga and Harold. Whereas Helga and Harold both have considerable amounts of depth, Sid and Stinky frequently display Lack of Empathy and turn on their so-called friends. Notably whenever an episode focuses on Harold, Sid and Stinky while be there to mock him until he finally gives a Big "Shut Up!" and threatens to beat the snot out of them.
Voiced by: Christopher P. Walberg
A kid from Arkansas with a thick Southern accent.
Catch Phrase: "This really bites." and variations of it was prevalent throughout the series. In seasons 3, 4, and 5, he also said "Willikers!"
Character Development: In Season One, he was more of a mindless lackey to Harold and wore spiky arm-bands. Later on, he was revealed to be a lot nicer and no longer wore the arm bands.
The Ace: Averted in "Ms Perfect"- the others girls think she's this, causing them to bully her, but then they find out Lila is very poor and no longer has a mother, making them have a collective My God, What Have I Done?.
Beneath the Mask: Not shown too much in the series, but in an interview Craig Bartlett said she had a hidden dark side. However, this calls for a reevaluation of pretty much all her scenes involving Arnold and his unreciprocated crush on her. Her dark side may have very well been there, just hidden under a veil of passive-aggressive sweetness.
Foil: To Helga, big time. A great deal of Lila's appearances, and the nature of Arnold's relationship with both, play up how Lila is everything Helga isn't.
according to Craig Lila had a repressed dark side. So in addition, while Lila was sweet and innocent on the outside, but emotionally disturbed on the inside, Helga was mean and crazy on the outside, but kind and sophisticated on the inside.
Love Revelation Epiphany: In "Arnold Loves Lila", Lila starts to like Arnold only because Helga wrote "Arnold Loves Lila" on the wall and Lila assumed it had to be true. Arnold tells her he doesn't "LIKE-like her" and Lila becomes very disappointed, but realizes she only came to "LIKE-like" Arnold because she thought he felt that way about her. So when Arnold realizes he does "LIKE-like her", Lila isn't interested in him anymore.
Manipulative Bitch: Not to an extreme, but there are times when, looking closely, you see Lila's rather sly. In a subtle way by expressions and choice of words. There's the question of her and Arnold's interactions, plus the fact that she reeled Helga in with no choice but to confess her secret rather impressively in "School Play". With Craig's claims of her having a locked dark side, it makes one wonder.
When asked about Helga and Lila becoming friends in the future (and if that meant that Helga accepts Olga as well) in The Arun Mehta Show Special With Craig Bartlett, which confirmed the idea was more of fan canon rather than actual canon that he's thought of, he had this to say -
Craig: Helga and Lila have a really complex relationship because Lila has helped her keep her secret and helpe—and Lila's a bit of a player, you know, she kind of manipulates people in her—in her sort of innocent way.
Master of the Mixed Message: Offers a sort of unintentional hope spot for Arnold sometimes, but ends up emphasizing at the end that she does not "like him like him"
Town Girls: The Femme to Helga's Butch and Phoebe's Neither.
Unrequited Love Switcheroo: In "Arnold Loves Lila", when Lila believes Arnold has a crush on her, she starts to like him too, but he "likes her, but doesn't LIKE-like her". Once she gets the message, he really does start to "LIKE-like her". Unfortunately, Lila decides that she now just likes him and insists in being Just Friends for the rest of the series.
Verbal Tic: "Ever so", "oh so", and "I'm certain that..."
Does This Remind You of Anything?: In "Curly Snaps", he, well, snaps, locks himself in the principal's office and throws basketballs at all his classmates who he thinks mistreated him for years. The other pupils react very shocked that the quiet Curly is capable of this.
Offscreen Teleportation: Being offscreen most of the time, it's pretty easy for Brainy to do this. Lampshaded in several episodes where he shows up behind Helga in the most improbable. places, such as on Elk Island, the Supposedly haunted train, and even in an Alley that Helga made sure was empty.
In-Series Nickname: Overlaps with Embarrassing Nickname. Most kids call her "Big Patty" even though as Harold says, "Her name isn't Big Patty, it's Patty!" Once people get to know and respect her, they drop the "Big."
No Guy Wants an Amazon: Played with. Sid and Stinky mock Patty because of her size, saying if they had to dance with someone like her, she'd probably crush them. But Harold, no small fry himself, likes Patty, not because he's an Amazon Chaser but because they have a genuine connection, one that leads to them spending all night just talking.
Proper Lady: Shockingly yes, if she would stop beating kids up. Then again she apparently only goes after people who have provoked her.
Cassandra Truth: In "Girl Trouble," he flat-out tells Arnold that Helga constantly teases him because she likes him. Arnold naturally doesn't believe him.
Cloud Cuckoolander: Not to the extent of his wife, but some episodes play this up to the hilt, such as the one where he believed himself stranded on a deserted island and confused a flock of ducks for his ancestors.
Generation Xerox: In "Girl Trouble," she and Phil had a relationship very similar to Helga and Arnold when they were younger. As part of the gag, Gertie in the flashback is voiced by Helga's voice actor.
OOC Is Serious Business: In "Parents Day," she acts like her usual fun-loving, crazy self. After seeing Arnold so depressed over his Parental Abandonment, however, she snaps out of the role she was playing and acts far more subdued.
Played for laughs during a heat wave; Arnold notes that the heat's so bad that even his grandmother is acting normal. Her wearing scuba gear and walking around like she does it every day when it starts raining is a sign that everything is going to be fine.
Helga's older sister and the parental favorite, much to Helga's dismay (and Olga's too, as revealed in "Olga Comes Home"). At first glance, Olga appears perhaps too perfect: Successful, pretty, smart, and sweet and kind to everyone. But she's actually dangerously neurotic, melodramatic and out of reality, due to having to constantly live up to her parents' (well, her father's, anyway) constant attention and ridiculously high standards.
The B Grade: Hasn't received an A- since third grade. In the "Olga Comes Home" episode, she cried for days in bed because she saw a B+ on her report card. Turns out, it was Helga who intercepted her college report card and changed her grade out of jealousy.
Broken Ace: She seems to be perfect at everything, but underneath her façade she's incredibly broken due to her father's absurdly high standards for her.
Cool Big Sis: In the aptly-named episode "Big Sis", she becomes one to Lila through volunteer work. With her biological sister Helga, however...not so much, but only because Helga can't stand her. However, Olga's come through for Helga on several occasions.
Olga: You must think I'm lucky, all the attention I get from them. I have to perform for them all the time like some kind of wind-up doll. I get really sick of it. You're lucky they don't even notice you.
Characterization Marches On: The latter characterization became steadily more common from Season 2 and onward. In the first season, he was portrayed as pretty much devoid of redeeming qualities.
The Dragon: To Scheck in the movie, in order to build a bigger Big Bob's Beepers. Though he defects from Scheck once he finds out that he's getting only 49% of the income.
Education Papa: Most of the pushing towards excellence that Olga received came from Bob's end, as he was the one who made it a big deal that she win so many awards and trophies, while Miriam just provided commentary. In the early seasons he even tried to shape Helga into following Olga's path.
Humiliation Conga: Helga was so disturbed by the "Orphan boy" line (see below) that during a gladiator-style event, she told Phil what Big Bob's weakspot was. Basically, she told him to go for the ankles, which sent Big Bob into a tub of gelatin - costing him the event and the overall win.
Basically any episode that accentuates his Jerk Ass qualities ends with him getting a solid block of hilarious karma dropped on his head. "Roughin' It" is another good example, where he ends up alone in his RV stuck with sunburn, swamped with bugs, and itching from poison ivy (not to mention having been electrocuted by his own faulty products earlier) while everyone else enjoys camping the old fashioned way.
He goes through another in "Summer Love". He sits in a lounge chair, gets sunburned, and has no one to take care of him, not even his wife whom he neglects. When he tries getting out of the house, he irritates his sunburn and gets a defeating blow from Miriam.
Jerk with a Heart of Gold: While he rarely shows it, he does love and care about Helga, and in some instances can help out other characters when needed. They even hint that he does care about his wife as well, especially in "Summer Love" when she told him about another man teaching her "the Watoosty".
Kick the Dog: His "orphan boy" line (referring to Arnold) in "Parent's Day". It's so mean it would be a Moral Event Horizon if he'd said it directly to Arnold (as it stands, he was talking to Helga and was just characteristically loud and uncaring about anyone else overhearing).
Took a Level in Kindness: "Big Bob's Crisis" was all about Bob realizing his high-stress life made him obnoxious, cruel and abusive to those around him - though he initially overcompensates by uprooting the entire family and becoming an Extreme Doormat, it ends with him deciding that he can be his old self while still being a little kinder and a little less stressed. Since this was his last appearance in the show, it's unknown whether or not he would've kept at it.
Wouldn't Hurt a Child: He may be a Jerkass like his daughter and use verbal abuse, but Bob draws the line when it comes to physically harming a child. In the Halloween episode he almost had Helga killed when he thought she was an alien, until he learns that "alien invasion" was just her in costume for a theme - at which point he was very distraught.
Mother of Helga and Olga, Miriam is a depressed, constantly sleepy, seemingly ditzy housewife with an addiction to "smoothies" (Word of God confirms that all of these are signs that Miriam is an alcoholic, with "smoothies" as a cover-up for booze). It's not uncommon to find her sleeping in strange places. However, she can be very competent and intelligent, but unfortunately it seems that being married to Big Bob has really brought her down.
Alcoholic Parent: While they couldn't state it outright (since Hey Arnold is still a kids' show, despite the myriad of "adult jokes"), Miriam is obviously a heavy drinker, with the show using smoothies as a metaphor for booze. If you look closely when she grabs some ingredients for her "smoothies", she grabs a bottle of Tabasco sauce and if "The Pataki's" was made, it is informed she would be attending AA.
All Girls Like Ponies: In "Road Trip," she reveals to Helga that when she was a kid, she wanted to learn to ride horses, but her dad refused to let her, fearing she'd get hurt. Her response to this, surprisingly enough, was to become a champion mechanical bull-rider.
Drives Like Crazy: Due to her drinking, Miriam has had her license revoked. When she took over for Bob's beeper store and drove Helga to school, all the kids were worried that Miriam was driving again (meaning that the kids know that Helga's mom is either a Drunk Driver or just a very, very bad driver).
Hidden Depths: She was once a world-class swimmer, won a bull-riding competition as a girl, and can run the family business better than Bob can if she wanted to.
Verbal Tic: Often speaks in a slow, drawling voice. Justified as Craig Bartlett has revealed that Miriam is an alcoholic.
Voiced by: Dan Butler
Fourth grade teacher at P.S. 118 starting in the second season, replacing the original Living Prop, Miss Slovak. Thinks of everyone and everything as "special".
Adults Are Useless: Specifically in the episode "Girl Trouble" when he doesn't punish Helga for bullying Arnold, yet he punishes Arnold for fighting back. Other examples include:
"Buses, Bikes, and Subways"—doesn't double-check his head count and is totally oblivious to the fact that two of his students are missing until the bus is back home.
"The Flood"—allows students to run riot for 3/4 of the episode and then tries to climb up a ladder during the storm for help, and ends up falling into several feet of water, meaning the kids have to save his butt.
"Aptitude Test"—mixes up two of the eponymous tests; doesn't bother to recheck the folders.
"On the Lam"—doesn't know his science closet was raided until informed on evening news.
Ambiguously Gay: Played by an openly gay actor, loves the theater, and has a very delicate, effete personality. In fact, a character who may or may not be his partner even appears in the Thanksgiving episode.
Cool Teacher: He can be this when he's not being useless. In fact, his premiere episode, "New Teacher," was all about the kids realizing this after they gave him a hard time.
Imagine Spot: In "Principal Simmons", Mr. Simmons imagines what the school would be like if he was the principal, which features the kids all beings friends with each other, Helga and Brainy walking together with their arms around each other, Wolfgang giving Eugene a piggyback ride, Harold becoming smart, the kids eating healthy food and happy to be the principal's helper.
Ink-Suit Actor: A very glaring example on a show with otherwise very outlandish character designs; Simmons strongly resembles his voice actor, Dan Butler.
Only One Name: It is strongly implied, but never explicitly stated, that his first name is Robert (in the Thanksgiving episode, he wrote a play that was based on his own life; we meet every character from the play in person later on except for him).
The CEO of Future Tech Industries, Scheck is a greedy and heartless businessman who wants to tear down Arnold's neighborhood to build a gigantic, immaculate shopping mall. Only appears in The Movie, where he is the Big Bad.
Affably Evil: Appears to be this at first, but it fades over the course of the movie.
Big "NO!": After he sees that Arnold, Helga and Gerald have escaped - with the videotape that exposes him burning the document.
Corrupt Corporate Executive: Not only is he not above burning a document declaring the neighborhood a national landmark, the whole reason he wants to build the mall in the first place is to get revenge after his family name was sullied over the "Tomato Incident" that happened in front of Sunset Arms.
Soft-Spoken Sadist: His voice is pretty unassuming and even somewhat soothing for a villain, but he tends to lose this when he gets mad.
Villainous Breakdown: He starts out appearing as an affable but very weaselly businessman, but ends up as a raving madman who is perfectly willing to run over several people, including children, with his car.
Only appearing in The Movie, Bridget is a sort of spy girl/superheroine who helps Arnold and Gerald save the neighborhood by supplying them with equipment and providing them a VCR to play the security tape exposing Scheck.
Ms. Fanservice: Both Arnold and Gerald immediately fall for her as soon as they meet her, and Gerald even faints when she gives him a good luck kiss.
When he finally does get a job delivering papers with Arnold's help, he's too lazy to even do that, making up excuses so that Arnold does it instead of him. It gets to the point that even Arnold calls him a "huge loser".
The Napoleon: Shorter than Arnold, but very passionate about the demolition business.
Tiny Guy, Huge Girl: He develops a crush on a very tall model named Lola, and the two even date until they had a falling out due to Lola being uncomfortable with their difference in height, though she apologizes, and they remain friends at the end of the episode.
Characterization Marches On: Mr. Hyunh is initially a stereotypical, high-tense Asian man who mostly panic and tends to yell the same thing over and over whenever there's trouble. Early into the show, this aspect of his character is lessened as they make him into more of a character and less of a stereotype.
There's also the episode where it's revealed he sings amazingly as a country artist. So much that Gerald was able to profit out of him for the episode before Hyunh chose "The Simple Things" of his life instead of the limelight.
Large Ham: How he responds to frustrating situations.
Mr. Hyunh: This lint. Your lint! I cannot keep cleaning up after you! I am not your mother! (beat) I'm not his mother!
In "The Vacant Lot", he was this at the end, shouting "Play ball!" 10 times in a row.
Mr. Hyunh: Play ba-all! Play ball! Play ba-all! Play ball! Play ba-all! Play ball! Play ba-all! Play ba-all! Play ball! Play ba-all!
Adventurer Anthropologist and Botanist Doctor: Perhaps to make them not be exactly like Indiana Jones they aren't Archaeologists, but have other jobs that would mean they might spend most of their time in a South American rainforest. But they still manage to rescue an artifact from a Jungle Pirate anyway.
Battle Couple: If Phil's stories and Miles' journal is to believed.
Mommy & Daddy Had A Good Reason for Abandoning You: They both left to help cure a deadly illness sweeping throughout the population of the South American country, particularly that of a secretive tribespeople who have helped them throughout the years who they owe their lives to. As Miles had written in his journal he expected the trip to be a short one... it is possible they too died of the illness in the end, though there is a Hope Spot when Arnold finds a map of their trail.
Parental Abandonment: Both left Arnold to be looked after by his Grandparents to depart on an important mission.
Screaming Birth: To give Stella her dues she may be angry at the time, but she manages to keep it together rather well for a woman giving birth in remote Jungle Temple whilst a volcano is erupting all around her!
Whip It Good: Miles is extremely proficient with a whip, using it to tether two cable-cars together at one point to stop one from falling.
Voiced by: David Wohl
As you guess, he's the principal of P.S. 118 and he's known to be harsh but has reasons to be one. Throughout the show, Wartz often is seen taken punishments too far, even by following teachers but he has toned this down after realizing when he sees how the others handle order.
Man Child: Shown to be this as he is seen playing with toys at his work desk and sending fan mail to teen singing sensation, Ronnie Matthews.
Pet the Dog: He can be reasoned with, even letting a kid off the hook depending on the situation.
He also has a set of toy animals on his desk in order to calm down and/or reduce the stress on any meeting he may have.
Properly Paranoid: Even if the kids have no interest in misbehaving, Wartz is often like this.
Stern Teacher: He’s harsh at times but he’s willing to negotiate if the situation can be resolved.
Willie the Jolly Olly Man
Voiced by: Dan Castellaneta
A mean, sadistic, downright crazy guy who works as an ice cream man.
Aesop Amnesia: Subverted. In Career Day, Arnold teaches him how to properly do his job and avoid getting fired. In subsequent appearances he flip-flops from being the same jerkass he always was, and actually doing his job well.
When Grandpa Phil accidentally runs him off the road in Grandpa's Packard, he goes as far as to dress up in drag, and trick Phil into giving his address, so he can steal the Packard from him. He apologizes after he gets arrested, though.
He makes a small appearance in the movie, though, where he seems to be good at his job again.
CloudCuckooLander: He's apparently convinced in Grandpa's Packard that the start of his bad day was his dog laughing at him.
Jerk Ass: He'll drive off without giving kids their ice cream, not give them the right flavors, yell at them and treat them with no respect, or just outright ignore them.
No Social Skills: He was not aware there was anything wrong with his attitude, and views the children he serves as frightening. When his boss/father shows up saying that his numbers are down, he says it's not his fault that kids these days don't want ice cream.
There was also a moment in Grandpa's Packard where he says that he was having a terrible day, one of the reasons being that the expiration date on his milk was wrong. He's not wrong, the milk was wrong. It'd be somewhat believable if it wasn't preceded by his dog laughing at him, and followed by a maniacal Grandpa Phil running him off the road on purpose.
Pet the Dog: He gives Arnold an 'A' at the end of 'Career Day' and a free ice-cream, as thanks.
Slasher Smile: Accidentally scares kids off with his smile when Arnold is trying to improve him.
Through the Eyes of Madness: We occasionally see how he views the world. In Career Day, he sees the city in black and white and nothing but slums, the kids as hyperactive creepy children, and his ice cream truck as a cage.
Grandpa's Packard shows that he views Grandpa's running him off the road as a malicious act against him, and not a mistake, complete with an evil looking Packard spewing flames and Grandpa Phil laughing maniacally.
A cousin of Arnold's who is disconcertingly similar, but much stranger and less attractive. Naturally, Lila falls head over heels for him.
Ambiguous Disorder: Arnie displays several signs of ASD, particularly Asperger's Syndrome, such as odd hobbies (counting things, reading ingredient labels [along with seemingly knowing what they are and how to pronounce them]) and collections (specifically balls of lint and "plain flavored" gum), displays muted outward signs of emotion, and seems to have difficulty in social interaction. These traits lead some of the other characters to think he is "dull" and "slow."
Bizarro Universe: The episode "Arnold Visits Arnie", in which just about all of the side character kids and main cast members get an eerily similar doppelganger with a completely opposite personality.
Mirror Universe: In "Arnold Visits Arnie", to some degree. That is, of course, if you believe that it isn't normal for Arnie to go Axe Crazy on his cousin and/or want to include the character alignments of the citizens of Arnie's hometown.
Murder the Hypotenuse: In "Arnold Visits Arnie" towards the end of the episode. It turns out it was just a dream, though.
Nightmare Face: Arnie gets one, also a result of the fiendish dream. Almost makes you wonder about Arnold a little...
Replacement Love Interest: This may be the reason why Lila was attracted to Arnie. Since Helga told her that she likes Arnold, she has agreed to let Helga have a chance with Arnold. When she saw Arnie, though, she saw the resemblance in their faces and pursued him in place of his cousin.