Kaitlin

• Kaitlin Cooper (played by Willa Holland) – (Seasons 3 & 4): Originally played by Shailene Woodley in a recurring role during Season 1, Kaitlin spends much of the series at boarding school before returning on a recurring basis in season 3 (now played by Holland) and finally becoming a regular character in season 4. The daughter of Jimmy Cooper and Julie Cooper-Nichol and sister to the late Marissa Cooper. After returning from boarding school, she begins to like Johnny, Marissa's friend, but when he dies she blames herself and goes back to boarding school. She comes home before Marissa is to leave for her boat trip and starts attending high school at Harbor during the fourth season. Her personality is more similar to her mother's rather than her father's, and quite different from that of her older sister Marissa. She dates Chris Brown who plays Will for a short time known as "band geek." In the finale, Kaitlin is shown congratulating her mother for graduating. She has a younger half-brother courtesy of Frank Atwood and Julie.

Taylor

Taylor Townsend (played by Autumn Reeser) – (Seasons 3 & 4): Introduced in Season 3 as a neurotic perfectionist student who battles Marissa for full control of Harbor High School's social scene. Summer and Seth become friends with Taylor after learning how lonely she is. In the season three finale, Seth tells her she has become inner circle material, making her a part of the "Fab Five". But shortly after that, Marissa dies in a car accident, thus making her the fourth member of the core four. In Season 4, Taylor comes back from France married and ready to be divorced and then becomes the love interest of Ryan Atwood. She is the daughter of the ill-spirited Veronica Townsend.

Jimmy

Jimmy Cooper (played by Tate Donovan) – (Seasons 1 – 2): Marissa's and Kaitlin's father (and Julie's ex-husband). He was a regular cast member for episodes 1–34, but his character left Newport for Maui. He dated Hailley for a while after his divorce with Julie. He returns following Caleb's death at the end of the second season and almost remarries Julie. After running into financial troubles (and being attacked), he decides to leave Newport again early in the third season. He lives on a boat for part of season 2, and as of the end of season 3, he is the captain of ship that is sailing in the Greek Islands. He returns in season 4 in the episode "The Chrismukk-huh?", but only in a parallel universe, where he is married to Kirsten, who he dated in high school.

Caleb

Caleb Nichol (played by Alan Dale) – (Seasons 1 & 2): Kirsten's businessman father and later Julie Cooper's husband. His character recurs throughout the first season, and he becomes a regular during the second season. Caleb is the show's first main character to die (from a heart attack). The reading of his will reveals that Caleb was broke, and leaves his widow Julie with nothing.

Julie

Julie Cooper-Nichol (played by Melinda Clarke) – (Seasons 1 – 4): The mother of Kaitlin Cooper and the late Marissa Cooper. At the beginning of the show she's married to financial planner Jimmy Cooper, but after learning that he had stolen large sums of money from his clients, Julie divorces him. She had an affair with her daughter's ex-boyfriend, Luke, while dating Caleb. She later marries Caleb Nichol and attempts to take-over as CEO of the Newport Group, with no success. Following Caleb’s death, she was left with no money and ends up living in a trailer park. She becomes engaged to Dr. Neil Roberts. The engagement falls apart after Marissa's death, and she becomes single again. Julie becomes involved with Frank Atwood in the fourth season and in the final episode, before Julie's attempted wedding to Bullit, it's revealed that Julie was carrying Frank's baby. In the finale, Julie graduates from college sometime in the future and she now has a son with Frank. She is originally from Riverside.

Luke

Luke Ward (played by Chris Carmack) – (Season 1 – 2): Marissa's jock ex-boyfriend and regular cast member episodes 1–24. Luke is the oldest of the three sons of Carson and Meredith Ward, and the older brother of Brad and Eric Ward (seen in season 4 being Kaitlin's "friends"). When Ryan first comes to town, the two of them get into constant fights, mainly over Marissa. The two of them eventually set aside their differences and become friends. He had an affair with Julie Cooper after he and Marissa broke up. The first main character to leave the show, Luke and his father moved to Portland after the people of Newport learned that his father was gay. He lets Seth live with him and his father in Portland over the summer break before school starts for their final year. This is shown in the first episode of season 2 before Seth returns home with Ryan.

Seth

Seth Cohen (played by Adam Brody) – (Seasons 1 – 4): The awkward adolescent son of Sandy and Kirsten Cohen. He is known for his quick quips and pop-culture references. He is also the love interest of Summer Roberts, on whom he had a crush since 3rd grade. He dated Anna Sternduring Season 1, but they break up due to a lack of chemistry. When Ryan moved back to Chino, Seth sailed to Portland on his sailboat and stayed with Luke and Luke's gay father in protest. Seth comes back home when Ryan returns to the Cohen household. After one of his break-ups with Summer, he had a fling with the bartender at the Bait Shop, Alex. He breaks up with her because her lesbian ex-girlfriend shows up, but eventually he realizes that he loves Summer. He lies to Summer in season 3 about getting into Brown, but only to make sure that she doesn't throw away her dream of going. The truth comes out eventually and he gets into RISD. He proposes to Summer in the fourth season, thinking that she might be pregnant. After he learns she is not pregnant, the thought of marriage begins to scare him and they engage in a game of chicken over who will be the one to terminate the engagement. In the finale, six months after a tragic earthquake, Seth and Summer take time to pursue separate interests but get married sometime in the future with Ryan as the best man and Taylor as the maid of honor.