Arnold is the main character in the TV show, Hey Arnold. He is the love interest of Helga Pataki.
Helga first met Arnold on her first day of preschool, when her parents neglected her. Helga pretty much had to raise herself, and Arnold was the only one who was kind to her,
when he held up an umbrella for her, while it was raining, and Helga had to walk to school due to her parents ignoring her. As a result, Helga fell in love with Arnold.
Helga refuses to let anyone know about her love for Arnold, because of fear that he will reject her. To avoid him from knowing, Helga bullies him, by calling him names (most famously, football head), and teasing him on a regular basis Helga has even gone to extremes to make sure nobody ever finds out. For instance, in one episode where she leaves a message on his home phone saying that she loves him, when she was under the influence of laughing gas, Helga actually sneaks into his house to steal the tape before he can hear it. Whenever Helga is alone, she takes out her locket of Arnold in a heart, and reminds herself of how much she loves him. Helga has also done other things in private to showing her love for him, such as building a shrine to him in her closet.
Although Arnold definately hates it when Helga bullies him, it has been hinted that he does like her. For instance, in the episode, Helga vs. Patty, Arnold tells Patty, who was planning on beating up Helga, that the reason Helga is mean is that she's insecure of her own feelings. Arnold has also admitted to liking Helga (perhaps more than a friend) when he's nice to him, in the episode Helga's Masquade. In the episode Arnold Visits Arnie, Arnold has a crush on a girl named Hilda, who looks exactly like Helga, except that she acts nice.
In the episode, Married, it is shown that Arnold and Helga will get married when they are older.