Thaddeus "Curly" Gammelthorpe is a fourth grader who attends P.S. 118 elementary school and the main antagonist (but sometimes an anti-hero) in Hey Arnold!. He is in Arnold's class. Curly is most well-known for his outlandish and bizarre behavior and freaking out to silly and trivial things (possible symptoms of psychosis or bipolar disorder). He is known as the "fourth grader most likely to snap". He is considered one of the geeks ("Rhonda's Glasses").
Curly is one of the more prominent side characters on the show, being a major character in a few episodes while generally the main antagonist and even the main character along with Rhonda in the episode "Curly's Girl". Curly is a petty, vindictive psychopath and will stop at nothing to get his revenge when he feels he has been wronged, even in ways that most people would see as extremely trivial. For instance, he once framed Eugene for pulling the fire alarm with an elaborate plan just because Eugene sharpened his pencil down to the eraser and left it chewed up (False Alarm). Curly's most well known act occurred in the episode "Curly Snaps". Because of a mistake in Mr. Simmons' plans, Sid became ball monitor when Curly was supposed to. Because of this, Curly stole all of the school's kickballs and locked himself in Principal Wartz' office. He continuously made demands for soda and threw balls at students and staff from the window on top of the door. It wasn't until Arnold was able to calm him down and Mr. Simmons admitted his mistake and set out to right it, that the problem could be solved peacefully. In "Ghost Bride" Curly is so outraged about not being allowed to tell the story of the urban legend, the Ghost Bride, so he dressed up in a wedding gown and came to the cemetery to try to scare the kids thinking that he was the real Ghost Bride. On at least three occasions (Ransom, New Bully on the Block, and April Fool's Day), Curly can be seen dancing ballet, having taken lessons at Madame Bovary's ballet school. It is unclear whether or not Curly is an animal rights activist, or if he just decided to do something crazy when he freed the animals from the zoo in "Cool Party" (since in the same episode it's also said he bit the head off a chicken).
- Curly is easily the most disturbed character in the Hey Arnold series.
- He appears to display either bipolar symptoms or some other form of psychosis.
- Curly has a crush on Rhonda, the latter whom easily reject his advances.
- Interestingly when she pretended to flirt with him, Curly grabs her and says "give daddy some sugar" (which suggests that despite being a child - he has an intimate side to him).
- At the end of the episode "Ghost Bride", it's unclear to what the Ghost Bride did with/to him, after Arnold and the gang left him in the tomb as revenge for pranking them.