In the episode "Road Trip", Helga turns on the car radio to a Hard Rock music station, suggesting that she enjoys music within the Rock genre.
In "School Play", Helga's mother, Miriam, yells into her bedroom and asks her if she remembered to take her constipation medicine in which Helga responds with a yes.
According to Sheena in "Phoebe Skips", Helga has a gastrointestinal disorder.
In "Student Teacher", Olga tells Mr. Simmons' students a story about a little girl who was a bedwetter overcame her problem. It is at the end of the story that Olga reveals to the students that the story was about Helga, who was a bedwetter until the age of 7.
In the Angry Beavers episode "Dumbwaiters", the beavers once showed Helga Pataki's mask within a set of scary masks.
Every time Helga signs her name on something that outwardly states her feelings to Arnold, it always ends up in Arnold's hands, leading her to take extreme measures to get it back or destroy it.
Youtuber Bo Burnham mentions Helga Pataki in the line "Hatin' other rappers like I'm Helga Pataki" of his song "Words, Words, Words".
From the show itself or comments from Craig Bartlett, it is affirmed that only four Hey Arnold! characters know of Helga's huge crush on Arnold. These four characters are: Dr. Bliss, Phoebe, Lila and Brainy. Helga has told Dr. Bliss, Phoebe and Lila her secret, but Brainy most likely knows because he stalks her.
It's also been implied other characters might know about her feelings. Phil speculates that Helga has a crush on Arnold in the episode "Girl Trouble", and Patty asks Helga if she and Arnold "have a thing for each other" in the episode "Helga vs. Big Patty".
Needless to say, Arnold himself knows about her feelings post-movie.
Helga said in the episode "Arnold's Valentine" that she likes a man who's honest "love-wise" (possibly because she doesn't want to end up with a dysfunctional family like Bob and Miriam).
Even though she wants a family who would pay attention to her, when they do she mostly tends to brush it off or isolate herself by saying that they're either embarrassing her or making her miserable.
Though Helga is mostly a tomboy, she has been shown to be feminine at times.
Helga may be mean, but whenever the situation is right, she can be characterized as caring, deep, and serious towards the people she's close to like Phoebe, Arnold (when she disguised herself as Cecile), Olga (once in a while) and her parents (to some extent).
In the episode "April Fool's Day", Arnold mentioned (when giving a gag present to Helga) that Helga's birthday was "last week," which means that her birthday falls on a day between March 19 and March 31 (which makes her an Aries). Among fans, Helga's birthday is often considered to be on March 25, the average date, or in March 26, Francesca Smith's birthday. This means Helga turned 10 in "April Fool's Day".
Helga seems to be a fan of Nancy Spumoni clothing and other items. Nancy Spumoni is a loose parody of Roxy clothing.
A Disney character, Isabella Garcia-Shapiro from the cartoon Phineas and Ferb, has a similar outfit to Helga Pataki's. Also Dan Povenmire who worked on Hey Arnold! is one of the creators of Phineas and Ferb creating another connection.
Grandpa sometimes calls her "your ugly friend with the one eyebrow" (in reference to her monobrow).
In the episode "Girl Trouble", Grandpa tells Arnold that he sometimes sees Helga skipping rope outside of the boarding house, waiting for Arnold to come out.
Helga appears to like the color pink, because throughout the show, almost all of her outfits are pink, including in a scene from "Married" where at the end, in which Helga and Arnold are married and she is wearing a pink dress. This contradicts with her image as a tomboy because pink is usually considered a girlish color. This could be because when Helga first met Arnold, he complimented her bow saying it was pink like her pants and she most likely adopted the color pink as her favorite because of this.
In the episode "Married" Helga has apparently blossomed into a lovely young woman, based off her dream of her married together and Phoebe's imagination (Which many fans have theorized to be a flashforward of them actually married) on what it would be like if Helga and Arnold were married. If this is what she will look like when she grows up, it seems she's just in an "ugly duckling" phase and will one day become a swan.