Tina, the "popular girl" at school, is the primary source of a lot of the bullying and name-calling that plagues Eleanor. Tina dated Park in sixth grade, and was his first kiss, which brought him a lot of social status in the process. Eleanor thinks Tina still has feelings for Park, which is probably why she sets out to make Eleanor's life miserable. Eleanor thinks Tina is "definitely a demon," with a "whole gang of lesser demons, all dressed in matching gym suits" (6.3) following her around.
Even though Tina is responsible for some truly horrendous acts of bullying—it's implied that she's behind a prank involving dumping Eleanor's clothes in the locker room toilet—she gets a moment of redemption at the end of the story when she actually helps Eleanor during the crisis with Richie. It goes to show that some demons are not as bad as others. And when Steve offers to kill Richie along with Tina's stepdad, we get the sense that Tina might know her fair share of hardship, too.