Arnie is considered weird by nearly everyone. His hobbies include counting things, reading ingredient labels on packaged foods (as well as somehow knowing what the ingredients are), chewing plain-flavored gum, and collecting lint. Lila has fallen in love with his "charms" (as well as Hilda, who appears in one of Arnold's dreams).
His head is an inversion of Arnold's (flat on top, round on the bottom), his skin looks paler, and his hair is a lighter shade of blonde than Arnold's. He wears a propeller beanie, a purple and yellow plaid shirt, black suspenders, green pants that rise above his ankles, white socks, and brown shoes.
Arnie is Arnold's cousin, though it is not said which side of Arnold's family he is from. Based on the shape of his head, as well as the fact that it was once mentioned that Arnold is Phil and Gertie's only grandchild, Arnie is most likely from Arnold's mom's side of the family.
In his first appearance, he arrives at the Sunset Arms to visit Arnold for four weeks. He goes to P.S. 118 temporarily, where he meets Lila, who immediately falls for him. Arnold, feeling jealous that Lila's more interested in Arnie than him, reaches an agreement with Helga to try to make Lila jealous, by pretending to be in a relationship. Their plan apparently succeeded but not on the right person, as Arnie falls in love with Helga and breaks up with Lila to be with her. He departs back to his home eventually, though the exact time is unknown; he may have stayed the full four weeks or left early after the trouble he caused and Helga's (presumed) off-screen rejection.
He later appears in Arnold's dream, wherein Arnold goes to visit Arnie, and encounters bizarre versions of his friends (ex. a flirtatious Lila counterpart, and many swapped friends, such as a dumb Phoebe and brainy Stinky). Arnie eventually catches on to the fact that his girlfriend Lulu is pursuing Arnold and confronts him. Arnie then becomes a more monstrous version of himself, and Arnold wakes up from his dream. It is unlikely that the residents of Arnie's town are as depicted in the dream, though it would not be out of the question.
- "Arnie" is also a nickname that Coach Wittenberg calls Arnold. However, he most likely does not know of Arnold's cousin.
- According to an interview with Craig Bartlett, "Arnie was a joke about when we get really ugly animation back from the studio. We turned that into an episode."
- It's possible that Arnie might have some form of Autism, due to his odd behaviors, hobbies, and tone of voice.