|Portrayed by||Tracie Thoms|
Prior to joining the squad, Kat had worked undercover, where she met and had a one night stand with a gang member named Jarrod Jones, which led to Kat's daughter Veronica Miller, whom Kat raises on her own.
While still working undercover in January of 2002, Miller was first on the scene when high school student and reformed drug dealer Skill Jones was fatally shot on the grounds of his school, dying in her arms. The incident would haunt her for five years until she was able to solve the case and arrest Skill's murderer, ironically an old informant of hers named Toomey Nelson.
Kat was once shot in the line of duty during an incident in Fishtown but recovered. She later claimed she'd made peace with it, though she attributed her heavy smoking habit to it.
Kat was working in Narcotics in late 2005, when she assisted the Homicide Squad in their investigtation of the 1973 murder of Carl Burton. She impressed Lieutenant John Stillman sufficiently that he offered her a position in the squad.
Despite some initial suspicion from Nick Vera, who noticed some notes on her desk regarding Veronica before learning this was her daughter, Kat quickly settled into the squad.
In 2008, the now-reformed Jarrod contacted Kat, asking to meet his daughter. Kat refused at first, largely out of fear of her daughter learning about Kat's past. She eventually relented, however.
Growing up in the dark, downtown neighbourhood of Kensington, she did not have the opportunity to receive a good education. She made the best of what she had, and still does.
In 2009, Kat was surprised when ADA Curtis Bell asked her out. She accepted, and the two would begin a quiet relationship after this.
In the fall of that year, Stillman was forced to transfer Kat to East Detectives under pressure from Deputy Commissioner Patrick Doherty, in retaliation for the squad's investigation of a case Doherty had ordered dropped. Stillman was able to his influence to get her returned to Homicide weeks later, however.
In early 2010, Jarrod and his new wife Chantel Jones moved back to Philadelphia and filed for joint custody of Veronica. Kat objected strongly to this and asked for Curtis' advice on how to proceed. Curtis attempted to be supportive but, having endured a ugly custody battle for his own daughter, soon became offended by Kat's steadfast resistance to Jarrod being in his daughter's life.
Kat and Curtis' relationship deteriorated after this, to the point where Kat did not attend the wedding of colleague Louie Amante, knowing Curtis would be there. However, in episode 7×20, the two reach a consensus of sorts, after Kat tells Curtis that Veronica sees her Dad "every other weekend" and that she seems happy. The end credits show Kat and Scotty Valens about to step into the same elevator as Curtis, but Valens stops at the last minute and walks away with a knowing smile on his face as the doors close on Kat and Curtis, implying that the relationship is back "on".