Colleen has long brown hair, "large, doll-like" eyes, and a petite figure. She is described as being curvy, yet lithe, and she wants to go into modeling.
Colleen sucked up to Hanna Marin and Mona Vanderwaal when she was a geeky freshman. One year later, she's still starry-eyed when she sees Hanna around. Unfortunately, Hanna never liked Colleen; she and Mona would make the girl do their homework and other random deeds for them when Hanna and Mona were still friends.
Hanna finally takes notice of Colleen again when she learns that Mike, Hanna's recent ex-boyfriend, is dating her. Colleen is still nothing but nice to Hanna, even when Hanna makes subtle insults about Colleen and Mike's relationship. Colleen is pleasantly surprised to see Hanna at a pole-dancing class she takes for fun at the local gym and offers a bit of advice. Little does she know that Hanna is trying to compete with her for Mike's attention. Hanna's not as good at dancing as Colleen, but she does get Mike's attention.
The next time the girls meet, it is at a marathon where Hanna and stepsister Kate Randall have set up a table for Hanna's father, Tom Marin's campaign. Colleen is as chipper as ever, but she's knocked down a bit when Hanna blatantly insults her.
Going on a tip from "A," Hanna decides to spy on Colleen. She follows the sophomore to a large home where a photographer, Jeffrey Lebrecque, lives. Thinking that Colleen might have fallen for the same trick Hanna had with a skeevy photographer who threatened her dad's campaign, Hanna peers through the windows to see what the two are doing. To her surprise, the photographer is legitimate and is earnestly taking expert shots of Colleen.
Hanna later tricks the older man into giving her the photos; however, they're all quite normal. She looks up Colleen's name online and finds her in a Latvian commercial for laxatives. Feeling like she has her trump card, Hanna plans to spread the video around the school.
At her dad's political event at the Gemological Museum, Hanna has second thoughts about sending the video. However, an encouraging text from "A" pushes her to do it. Ultimately, Hanna decides not to send the video, prompting "A" to do it for her. However, it is not Colleen's commercial that is spread, but a video of Hanna following her around in multiple locations, including Jeffrey's home. Colleen is now legitimately freaked out by Hanna when she sees the video.
Despite Hanna's new reputation as a stalker, Mike later breaks up with a tearful Colleen and decides to get back with Hanna, flattered that she did so much to try to win him back and admitting that he didn't even like Colleen that much, confirming the rumors that he had slept with her to be false.
in Deadly, she sends Hanna a text saying "I should have known you were capable of this kind of thing," after the fake video of the Liars killing Tabitha was released.