These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Also Arnold himself at the end of the episode "24 Hours To Live".
Curly, with the emphasis on crazy (and not a "cool, kooky" crazy either. More like a "This kid needs serious mental help" crazy). To elaborate, he once broke some animals out of a zoo - alone. Just because he wanted to.
Chocolate Boy can be this, too. Apparently, he's spent so much time around chocolate that he can be used to literally sniff out Helga and Harold after they get separated following a tour at a chocolate tour (made all the more impressive, because this was several hours after the duo had been lost). Made funnier because he's literally put on a leash.
Draco in Leather Pants: Helga. In the series she is a Jerkass Woobie who is horribly neglected by her parents, but who also can be mean, selfish,rude (and even cruel in some instances) does many things moved by jealousy, constantly bullies other kids (Not only Armold) However, despite that, many fanfics (specially romantic fanfics) portray her almost as a saint.
I'm going with the idea that Arnold considers the mooning, while still offensive, is less so than betraying his friends; he might have said something to them later about not doing it again. He does have a strong sense of honor and probably would have spoken up had they been stealing or vandalizing school property. But it's still a bit disappointing.
Fan Fic Fuel: We have enough details about the Jungle Movie and The Patakis that many fanfic writers have written their own versions.
"Funny Aneurysm" Moment: At the end of "The Flood", as floodwaters surround PS 118, Principal Wartz stands on the roof of the school singing, "Way down yonder, in New Orleans..."
In "Helga on the Couch", Mr. Simmons gets nervous when he learns that the school psychologist will be observing is class, stuttering and asking for reassurance that she will just be observing the students, and not him, the teacher. While this is a moment played for laughs when the viewer is a child, as an older viewer who knows about the above-mentioned Word of Gay, he was likely still in the closet and worried that the school psychologist would see through him and expose him, which could have disastrous backlash from the parents whose children attends P.S. 118.
Harsher in Hindsight: It was once implied that Helga's dysfunctional home life was the end result of her sister Olga moving out (as her parents favored Olga over Helga), leading her dad to push Helga to be more like her older sister while Miriam became a covert alcoholic (or "smoothie" drinker) just to cope with dealing with a Jerk Ass like Bob and a bratty daughter like Helga. The episode "Helga on the Couch" revealed that Helga's been neglected by her parents since she was preschool-aged and that her secret crush on Arnold was the first time anyone has given her any sort of affection in her life.
The episode "Rich Kid" involves the new kid Lorenzo who has a cell phone, beeper, his own laptop, etc and is basically a mini adult who needs to act like a kid. Many kids in the 2010s have their own cell phones, tablets, computers, etc and one complaint among older people is that kids are growing up too fast more than ever.
Jolly Olly Man, the ice cream vendor though this troper had a hard time understanding his frustration caused by only a boring job, and his problems keeping it which may have something to do with his jerkassery which delves into Nightmare Fuel. It's not good for the business if you act like a raging psychopath even with your customers. And he had no idea what's wrong with his behavior. Arnold had to teach him the basics of common courtesy.
Memetic Mutation: "STOOP KID'S AFRAID TO LEAVE HIS STOOP! STOOP KID'S AFRAID TO LEAVE HIS STOOP! STOOP KID'S AFRAID TO LEAVE HIS STOOP!"
Never Live It Down: Iggy. In most episodes, he's simply a background character, with only one or two notable lines. "Arnold Betrays Iggy", his A Day in the Limelight, will forever have him be known for all of the (well-deserved) negative tropes on this page.
The Scrappy: Lila toward the end of the series. While a few people have come to her defense against some of the harsher attacks in recent years, you'd be hard-pressed to find people that are honestly fans at this time. Inicially she does/did have a fanbase for her appearances in Seasons 2 and 3 (hell, she was actually The Woobie when she first debuted!) It was mainly due to the Romantic Plot Tumor love triangle of Seasons 4 and 5 (and accomponying Die for Our Ship sentiments) that pushed her into Scrappydom.
Iggy from the episode "Arnold Betrays Iggy". It doesn't help that the episode itself ends on the note that Iggy has gone beyond Arnold's forgiveness.
Gerald's little sister Timberly gets a lot of hate in fandom mainly for being really annoying and is usually hindering Arnold and Gerald's plans in any episode where she plays a key role. The best example of this is the "Chocolate Turtles" episode.
Sid. He is a Poisonous Friend who betrays everyone at the drop of a hat, makes fun of Harold and Patty like a Jerk Ass, and everyone forgets he pulls the same crap over, and over, and over again.
Also, Big Bob for being a terrible father and an all-around terrible person towards his daughters and wife.
Olga herself: Despite showing Hidden Depths and admitting she'd rather have the life that Helga has...she hasn't really shown it since that one episode. She comes off as so blind to what her sister goes through, continues to perform her perfect act for her parents (despite saying she hates it, making her a Hypocrite). Also she shares embarrassing stories with Helga's class despite promising not to, making her a liar to boot.
Seasonal Rot: Seasons four and five aren't considered fan favorites, mainly due to Arnold's character becoming bland and the Arnold/Helga/Lila love triangle becoming a Romantic Plot Tumor (contrast to Season 3, where it only came up twice and seemed to be resolved within those two episodes!) There were still some good episodes here and there, though, particularly "Parent's Day" and "The Journal."
Ernie comes across as an obsessed stalker in "Ernie in Love" before he asks Lola out. He's doing the same thing Helga does (he follows her, can't tell her he loves her, and has a statue in her room) Helga's a kid, which is more understandable and innocent, but he's a grown man. It just comes across as creepy.
Wangst: Olga is a master of this. In "Olga gets a B", she spends days sobbing about it in her room, with dramatic music in the background and a serious case of Your Makeup Is Running. Even Big Bob starts to get tired of her histrionics- she only snaps out of it when Helga admits she changed her grades out of spite.
What an Idiot: Helga probably could get Arnold's interest if she was nice, but she rarely gives it a try. She has a rep to maintain. Harold even made fun of her on day one (when Arnold first noticed her) in a flashback in the episode, "Helga on the Couch".
Arnold realizing that Iggy won't forgive him and instead of letting go, he suffered through a Humiliation Conga.