Hey Arnold!

Mr. Simmons, Arnold's empathetic teacher, is a gay character. In the episode 'Arnold's Thanksgiving,' Mr. Simmons invites friends and family, including Peter, to his Thanksgiving dinner. When Mr. Simmons' mother expresses surprise at Peter's presence, Mr. Simmons replies, 'There's a lot of things you don't know.' Creator Craig Bartlett later confirmed that Peter is Mr. Simmons' partner, and Mr. Simmons himself is gay. Peter also makes a brief appearance with Mr. Simmons at the airport in 'Hey Arnold!: The Jungle Movie,' where they share a hug.

In 'Mugged' (Season 1, Episode 11), Arnold is mugged, leading his Grandma to teach him self-defense. Arnold subsequently overuses his new skills, accidentally harming an innocent man, which highlights the unintended consequences of violence. In 'Curly Snaps' (Season 3, Episode 6), a character named Curly takes several balls hostage and barricades himself in a room, leading to a tense, almost hostage-negotiation-like situation. 'Hey Arnold! The Movie' received a PG rating partly due to 'some violence,' including characters resorting to less conventional, and implicitly illegal, methods like packing stolen explosives to prevent demolition.

Helga G. Pataki harbors a profound, often obsessive, secret crush on Arnold, which manifests in grand gestures, extensive poetry, and a hidden shrine dedicated to him ('The Little Pink Book,' Season 1, Episode 6). Characters sometimes engage in romantic interactions that reviewers note are mature for their age, such as attending school dances with dates, like the Valentine's Day dance where Arnold has two dates in 'Arnold's Valentine' (Season 1, Episode 20).

In 'The Haunted Train' (Season 1, Episode 2), Grandpa Phil tells Arnold and Gerald a scary story about a haunted train and its mad conductor, leading the children to investigate and encounter frightening, though ultimately explainable, phenomena. Another notable episode is 'Ghost Bride' (Season 5, Episode 20), where Gerald recounts the urban legend of Cynthia Snell, a bride who brutally murdered her fiancé and sister with an axe before taking her own life, and is said to haunt a cemetery.

Miriam Pataki is frequently shown disoriented, slurring her words, or excessively blending 'smoothies,' which are clear allusions to alcohol consumption, a fact later confirmed by creator Craig Bartlett. The character Chocolate Boy (Season 5, Episode 10) is depicted with an extreme, addictive craving for chocolate, exhibiting withdrawal-like symptoms when deprived. Arnold even uses a slideshow of negative consequences, including vomiting and jail, to illustrate the dangers of addiction to Chocolate Boy.

Episodes like 'The Haunted Train' (Season 1, Episode 2) are cited as genuinely terrifying, featuring eerie stories, creepy atmospheres, and high tension as the children investigate a supposed ghost train. In 'Part-Time Friends' (Season 3, Episode 16a), a nightmare sequence features an 'undead Grandpa Phil' reminding Arnold of his mistakes, which is described as a creepy and unexpected image.

Helga frequently shows outright disrespect and defiance towards her parents, Bob and Miriam, and occasionally towards teachers or other adults, often through sarcastic remarks or direct challenges to authority. In 'Hey Arnold! The Movie,' Arnold and his friends resort to illegal activities, such as employee theft and using stolen explosives, in their efforts to save their neighborhood, which a parental review notes as problematic for suggesting such methods to elementary-aged children.