Her stepfather,
Paul Brownlee, is the primary suspect in her disappearance. She reported that he was abusing her. Her older sister, who moved to California in 1977 to live with her biological dad, reported the same of Brownlee, and he actually pleaded to a "Fourth degree sexual conduct" charge with respect to her. Kellie was living with her boyfriend's family to be away from the stepdad, and the boyfriend was the one who sounded the alarm on her being missing, and was eliminated as a suspect. She had accompanied him to school, but apparently went to a local mall to apply for jobs, where she was last seen.
The stepfather agreed to be interviewed by police, denying having abused her or had anything to do with her disappearance. He also called in a tip at some point, claiming that she'd been seen in the company of two men at a local ice cream parlor before going missing. After his interview with police, though, he lawyered up and refused a polygraph.
One of the interesting things about his "tip" to police is that it might have been inspired by the disappearance the previous year of
Kim Marie Larrow, in Canton, OH, the previous year. Kim was supposed to have been meeting up with her party gang in a park after seeing them in a local ice cream parlor, but never made it. Her best friend worked at the parlor, and planned to meet up with her at the park.
I'm assuming the ice cream parlor would have been mentioned in reports of Larrow's nearby disappearance before the time that Kellie went missing. Chances are that Paul Brownlee had nothing to do with Larrow's disappearance, and that he was just trying to throw police off his scent, but in offering up the supposed tip about Kellie he seems to have made himself a potential suspect as well in the disappearance of Kim Marie Larrow.
Paul Brownlee seems not to have been particularly bright. A detective who took over the case reports that shortly after he did Brownlee appeared with a catalog, claiming that a model in a hot pink bathing suit was his missing stepdaughter, which seems pretty transparently an effort to size up the new guy. The new detective asked him where he was on the date of her disappearance, and he split.
With a conviction regarding her sister, and Kellie showing friends the bruises she'd sustained from fights with Paul Brownlee, he probably decided that a court wasn't going to be so lenient again if he was reported and charged. Brownlee's mother didn't divorce Paul until 3 years later.
Kim Marie Larrow and Kellie Brownlee were both party girls, both had similar Farah Fawcett-inspired hair cuts. Larrow was about the same age as Brownlee's sister at the time she was abused by her stepdad. If Paul Brownlee got rid of his stepdaughter it's likely he did it to prevent her talking. If he had something to do with Larrow's disappearance, it's likely he would merely have abused her, but couldn't afford to get caught again. His prior in the matter of his other stepdaughter probably made him more dangerous than previously he had been. He seems to have been very impulsive.