Phoebe Heyerdahl is one of supporting characters of Hey Arnold! series, two films. She is among the smartest children in the school (she was given a free pass to the 6th grade in "Phoebe Skips").
Her distinctive features include a huge lump of black hair at the top of her head bounded by a small yellow scrunchie, and stereotypes for smart children in animated shows, such as big eyeglasses (in Phoebe's case, with blue monochromatic lenses), a short stature, and squeaky voice. She always tries to be the first one with the answer, and isn't exactly thrilled when someone else does better ("Ms. Perfect", "Phoebe Cheats").
Phoebe was born in Kentucky and, at some point, moved to Hillwood at a young age.
Among her most notable moments was when she won the City Academic Bowl ("Phoebe Takes the Fall"), a competition that she shouldn't even have been in, as Helga, in an attempt to win the only trophy her older sister Olga never won, had won her school's spot at the Bowl by forcing Phoebe to miss the final question (ironically, McAfee did not receive credit for the role in that episode). At the opposite end of the spectrum, she was utterly humiliated in front of the entire student body ("Phoebe's Little Problem") when she was caught passing gas into a microphone on stage as she was receiving an award, and subsequently refused to ever go to school again (somewhat ironically, the award was for her perfect attendance). Among her other achievements are winning the Parents Tournament Weekend with her parents ("Parents Day") and the City Go-Kart Grand Prix with Helga and Harold ("Grand Prix").
She's also pretty good at handling a fencing sword ("Phoebe Cheats"), even when it's a flashlight ("Wheezin' Ed"). There is some evidence that she is musically inclined as well; she's carrying what appears to be a string bass in "Helga on the Couch" and is practicing some sort of large horn in "School Dance". (However, music is one of those disciplines where you need creativity, and that seems to be one of few things that Phoebe lacks, if "Phoebe Cheats" is anything to go by.) Just as Gerald hangs around with Arnold, Phoebe hangs around with Helga, who considers Phoebe her best friend (she even says so in "Magic Show", after yelling at her), mainly because Phoebe is probably the only person who likes Helga the way she is ("Helga's Makeover").
At first, Phoebe was Helga's "shadow", right down to the point where seeing Helga cry made her cry as well ("Downtown As Fruits"), but she's starting to "let go", and it may be in part because she's developing a noticeable crush on Gerald.