Heather is the love interest of Oz in the American Pie films.
Heather was part of the school's choir. She comes across as a goody-goody, but given how obsessed with sex all the other characters are, this isn't a very strong statement about her.
Oz was initially just a school jock who joined the choir group so that he could sleep with the girls. It wasn't until he got to know Heather that he began to develop real feelings for her. When Heather suspects that Oz's original intentions, he auditions for the lead in the upcoming school performance to show how committed he is. Oz's priorities are put to the test when he finds out that the playoffs are on at the same time as the performance. In the end, he picks Heather over everything else. Seeing how much Oz was willing to give up for her, Heather goes to the prom with him and they later make love.
Oz and Heather go to different colleges but go steady. Oz barely notices other girls. Over the summer, Heather goes abroad while Oz stays at a beach house with his friends. They call each other regularly, but Stifler keeps eavesdropping so they decide to wait until Heather gets back. Heather arrives in time for the gang's beach party.
It's revealed that in the following years Heather and Oz split up and started dating new people. Oz still kept a torch for her up until their high school reunion, where he admitted that he still loved Heather. They eventually get back together.
Ron was Heather's new boyfriend in the Reunion film. He's aware of Oz's history with Heather, but isn't too bothered by it. Upon noticing Oz's new girlfriend, he suggests they have a foursome. This is a key indicator that he doesn't love Heather nearly as much as Oz did. When Ron finds out that Heather shared a kiss with Oz, he tries provoking Oz into attacking him, threatening to sue if he does. Stifler punches Ron instead.
Mia is Oz's new supermodel girlfriend. She's shallow and only likes him for his success, while Heather liked Oz for who he was. She catches Oz kissing Heather and attacks her. Oz happily breaks up with her after after that.