Petra is the lone female member of Silas' criminal gang, and it was revealed that (at least) 16 years prior to the film's events, she participated in a robbery which had LAPD detective Erin Bell and her partner/lover, Chris, infiltrated in the gang as part of an undercover sting. The robbery went sideways due to Silas shooting and killing a bank teller and later Chris (who went in following the first shooting), after which Silas and Petra handed their ill-gotten gains to lawyer Dennis DiFranco to launder. It was also revealed that Petra and Dennis had engaged in a romantic fling during that period.
The film's events had Petra, revealed as a drug addict, participating in another robbery, as the money from the previous heist ran out. The latest robbery featured a shootout between the gang and Erin, with Petra shot in the leg but managing to get away and ditch her disguise. However, the evil Petra ended up spotted by a young boy and later by Erin, who pursued the villainess and brutally fought her before placing Petra in the trunk of her car. Petra lashed out at Erin and accused her of wanting the money as much as she and the others did, which ended up revealing that Erin became a willing participant in the robbery (as did Chris) after finding out that she was pregnant with their daughter, Shelby.
Under Erin's orders, Petra gave more info on Silas, and the end of the film revealed that Erin suffered internal bleeding from the fight and passed on as a result. Before her death, Erin gave her partner, Antonio, the location of Petra, which resulted in Petra's off-screen arrest.