Joey Grant was a cop for the Boston Police Department, once working for Narcotics before being promoted to lieutenant for Homicide. He and Jane Rizzoli have known each other since they were kids (since he was 5 years old, to be precise) and have had a complicated relationship since then, wherein they both appear to always be at each other's throats, although they are evidently attracted to each other.
During their days in school, Joey seemed to always bully Jane by calling her names like "Frog Face" and "Rolie Polie Rizzoli", and once even put a mallard duck in Jane's locker.
Meanwhile, Jane always describes him sourly, calling him a brass-kisser to get into convenient positions, and, among other things, a cheater because of a catechism test when they were 8 years old that Jane was never able to let go (although Joey later reveals that he was staring at Jane and not at her paper).
He temporarily became Jane's boss at homicide, much to Jane and Korsak's chagrin. At first, he taunts Jane a lot with his position, threatening to get her off cases if she doesn't start respecting him. After Jane solves the Boston Strangler Redux case while Joey followed a wrong lead, the hostility between the two seem to have lessened. When Jane was threatened with snakes on the case she was following against a priest, Joey believes her theory and asks her to be careful, evidently worried about her well-being and how she will handle the case properly.
In Sympathy for the Devil, Angela Rizzoli sets him and Jane up on a dinner date at their house. Although Jane is initially infuriated with the setup, the two get along after their first glass of wine, but after the wrong choice of words are exchanged, the two decide that to call off dinner and they immediately go back to their bickering ways.
Later, it is revealed that he has been reassigned to Washington D.C. to become a liaison between Boston and Homeland Security. He waits for Jane in the rain outside of her apartment to bid his farewell and he confesses that all the bullying he had done to her when they were kids was because he had already liked her since and never really stopped. About to kiss, Jane recoils, saying that he is leaving. She tells him to have fun in Washington and that she will miss him before kissing him twice on the cheek and walking away flustered.