Riley Cooper is a detective that became Jane Rizzoli's neighbor at one point.
She moved into the building sometime before "Cuts Like a Knife", later revealed to be on purpose because she feels safer living near a cop, which was when Jane had just bought a new bed. Frost and Frankie were supposed to be helping Jane but they both get sidetracked and help Riley move in her things instead. Angela, trying to be the nice neighbor (or at least mother of the neighbor), decides to give Riley some of the things she gave to Jane, pissing Jane off even more.
Frost and Frankie compete for her attention. Later, Riley dates both Frankie and Frost separately at the Dirty Robber and is seen by Jane and Maura. Frankie asks Jane not to interfere because he really likes her, but Jane tells him that she finds Riley suspicious. However, it turns out that both Frost and Frankie knew and were just waiting to see who Riley would choose and sleep with first.
One night, Jane overhears Riley, outside her apartment, make a deal for 5,000 liquid units, and Jane deduces that it is for liquid ecstasy. She then looks her up at the station and confirms that she is involved with drugs. The drug unit tracks her search and they ask Cavanaugh to tell Jane to not make contact with Riley because she is involved in an ongoing investigation. Jane then tells Cavanaugh about Frost and Frankie casually dating her and suggests warning them, but Cavanaugh says that they can't because they do not know what the drug unit has planned.
The next day, Riley and her seller are caught making their exchange and are arrested by the DCU. Jane, Frost, Frankie and Angela all happened to be at the station and saw her being brought in.
In "Melt My Heart to Stone", it is revealed that Riley was in fact part of said bust and was actually an uncover DCU detective who had set up the operation for a year. She is then promoted to the homicide unit, after five years in the drug unit, over Frankie. Both Frost and Frankie are furious for being used by Riley for the promotion, while Jane and Korsak are a bit more empathetic, having been undercover cops once themselves and knowing how hard it could be. When Riley tells Jane that she actually does like Frost and Frankie, Jane advises her to find a way to make things right.
To help their case, Jane later asks her to help I.D. their victim through her sources, and has Frost and Frankie go with her. Despite the boys' grudge, they get a breakthrough after she asks the right prostitute about their victim. Later, Jane introduces Riley to senior criminologist Susie Chang.
Detective Riley is then borrowed back to the drug unit, as revealed in "Class Action Satisfaction".