Nina Harper is the school's very beautiful queen bee who bullies Sharon and her friends.
Nina sports long red hair, which according to Sharon is not her natural color, which is brunette. So she colors it for beauty and vanity purposes.
Sharon and Nina were once close friends when they were kids, but that all changed when Nina found her dolls with their heads chopped off and blamed Sharon, even though it was her cousin Petra as revealed in the episode "Clean Slate", despite Sharon pleading that she did not do it. Ever since this incident, Nina has been doing her best to wreck Sharon's life and those who dare attach themselves to her. For example, Nina was the one who stuck Sharon with the nickname "Braceface" when she first got braces.
Despite her beauty, Nina is not well liked by many kids, but loved by adults. Although many people do not like Nina, there are some who would not mind being her since she has the money, looks, and life a lot of people would love to have. It is revealed in "Up in Smoke" by Alyson, that she and Nina used to smoke, but were caught by their parents and both found it really hard to quit.
Nina has helped Sharon on a few occasions though like in the episode "Lorenza" when she helps Sharon look for the dog Lorenza when it goes missing. Later on, Sharon gives Lorenza to Nina when she revealed why she wanted a dog. Nina's aspiration in life is to be an international fashion model. Part of this reinforces her attitude as a heartless bully who subjugates those she sees as flawed to ridicule and pull spiteful pranks on, Sharon and her friends for example, in the episode, "Triangles" Nina made a fake humiliating slide show of Sharon and Maria for turning Alyson against her.