Hillary is "BFF" or best female friend with Lori, and also occasional enemies.
Before Season 1
Hillary's reputation as a slut began during her 8th-grade school field trip to Tacoma; details were not explained.
Hillary attends Declan's party and meets Lori there, warning her not to notice Declan with another girl. She sees Kyle defeat Officer Wolner and then carry Lori home, since Lori had passed out drunk. The next day, she asks Lori more about "your knight in shining armor....the guy with all the Jackie Chan moves?" She tells Declan that Lori and Kyle are an item. Lori later corrects that, not wanting to put off Declan's interest.
At Jeff Preston's party, Lori calls Hillary a slut for laying in Kyle's lap. In retaliation, figuring out that Lori just gave up her virginity to Declan in the bushes, Hillary says "Who knew after all this time, you'd be so easy." At school, Lori tries to gossip about another girl, but Hillary ignores her. Their friendship is at an end, with Hillary making their group avoid Lori, and spilling a drink on Lori's blouse which is the same as Hillary's.
They decide to put their feud aside as they realize it isn't worth it, they both need each other too much. In the Overheard episode, Hillary tells Lori and Amanda that she has a new boyfriend, but won't say who. It is later revealed to be Charlie, Amanda's boyfriend. At Kyle's going away party, Hillary overhears Charlie tell Kyle that he is only using Hillary for his physical needs. In response, Hillary splashes soda in his face, and ends their relationship. Lori later comforts her by saying that she will find someone who loves and deserves her.
Hillary and Lori decide to "revirginize" in an effort to simplify their dating problems. While practicing by doing what Amanda does and volunteering to help with the Bonfire Party, she and Lori convince Amanda not to have sex with Charlie. Hillary later confirms to Amanda that she is part of the reason Charlie is voted the school's Biggest Slut.
Hillary and Lori explain to Kyle that going to the Spring Fling dance with Amanda means "You, have a date." But Hillary and Lori are refused the cheaper couples' tickets for the dance. They join Andy Jensen in her protest against Principal Hooper's rule against same-sex couples at the dance. At the Swing Spring Fling they have at The Rack, Hillary and Lori tease Charlie by kissing.
At the cancer charity Hands on a Hybrid event, Hillary goes around with her cameraman documenting the competition. Hillary calls it "investigative journalism". Lori enlists Hillary's help in finding dirt on Jessi by reviewing all the footage she has. They learn that Joshbought a ticket chosen to be in the competition to try to help Andy. With Declan's help, they also find out that it was Jessi was the one who attacked Lori at the Alterna-Dance.
Hillary comes to help Lori plan her revenge against Jessi, but quickly becomes annoyed by Lori and Declan's constant arguing. She decides to play psychologist, but complains to Nicole that she's no good at helping people. Nicole reassures her that she is not "a dumb blond".
Hillary continues with her camera journalism when school resumes. She has her reports on a blog, and is infuriated with what seems to be someone in the background jumping off the school building unharmed. Hillary calls "the mystery jumper" a fraud in her next video. Jessi runs across a swimming pool in Hillary's next fashion broadcast, staying there long enough for her face to be seen. Hillary turns her report about an impromptu party on the school roof into a show-down with Jessi about her jump. Only Kyle's special efforts prevent Jessi from jumping, and that puts an end to her notoriety.
Taking a surprise part in the date Kyle arranges with Amanda at The Rack, Hillary serves them a very special French meal which no one knew she could do. She also helps Stephen save the anniversary dinner he had planned for Nicole by making her special Provence salad "with drizzled leeks." While he tutors her in geometry, she tries to explain that Lori is meant for Declan, and dating Mark is tampering with the laws of nature.
Hillary's part in preparing for Prom night is arranging for a French caterer whom she knows. Having dated every desirable boy in the high school, Hillary's "date" for Prom is the Queen of the Prom tiara. She campaigns shamelessly with various flyers. She doesn't win, and ends up comforting Declan at the "after-party", since he and Lori broke up at the end of the main event. They start by toasting with champagne she got from the chef in return for an ad at her blog. Declan becomes so drunk later that he kisses Hillary, and they are seen by Lori, who is then mad at both of them.
Before the big Street Fair, Amanda arranges a night without Kyle at home with Hillary instead. She also invites Lori, not knowing about their fight. When Jessi shows up, sent by Kyle to watch over Amanda, Amanda asks Hillary if she still has tickets for the band concert, and they go off. Jessi is not supposed to join them, so Lori does. She finds them at the fair, but Hillary tears up her ticket. They start a fight using squirt bottles of juice for ice cones. As in the prediction by the gypsy fortune teller, when Declan arrives he is "caught between enemy lines."
Declan separately invites Lori and Hillary to The Rack for "some quality friend time" with him. Instead, he locks them in the back room, promising to let them out when they have made up. They refuse to speak at all, so he takes in some soda and aerosol cans for them to fight with, as they had at the street fair. Still they won't quite get started. They see him with Jackie, his new interest. They resolve to get back at him by escaping through the roof panels to wreck his date. They do get out, but land in the garbage dumpster. Lori calls Declan to lure him into the room, and they lock him in. Both still soiled in garbage and Hillary limping with a broken shoe heel, they advance toward Jackie, whom Hillary ironically calls "cute little blond thing." When Jackie goes to release Declan, she gives him a big kiss, because in telling her about Declan, Lori and Hillary had only told her how wonderful they both knew him to be.