Lilly Rush was the only female homicide detective in Philadelphia, until the later arrival of Lennie Desalle, then Josie Sutton, and finally Kat Miller. She specialized in working cold cases alongside her partner, Scotty Valens. According to Lilly, "People shouldn't be forgotten, even if they're my kind of people. Maybe, they don't have a lot of money, they don't have lawyers, but they matter." Shortly after Lilly's change from on-the-line jobs to cold cases, her partner, Chris Lassing, transfers out because of his diabetic condition, leading to Scotty being promoted to homicide and becoming Lilly's partner. Lilly solves the most difficult cases in order to seek the truth after all these years in hopes of giving the victim's family and the victims themselves, justice. She often works long hours on these cases. Because of this, her relationship with men are relatively shaky and her family can best be described as dysfunctional.
Lilly was raised on welfare by an alcoholic mother, Ellen Rush, who often neglected her. This left Lilly to fend for herself and care for her younger sister, Christina Rush. The family lived in Kensington, a rough part of Philadelphia. At the age of ten, Lilly had been sent out late at night by her mother to buy alcohol, and a man brutally attacked and robbed her. Suffering a broken jaw, among other injuries, young Lilly was nursed by Ray Williams, whom she later almost married as a young adult, but neither could bring themselves to get off of his motorbike to go into the courthouse after having ridden to Knoxville. Later in life, Lilly confides to her boyfriend Joseph Shaw that she was once engaged to Ray, but he was not the "settling down" type. It is Lilly's lack of trust in other men that leads her to be alone most of the time. Her lack of trust can be traced back to her childhood when her father walked out, and also when Patrick, a man Lilly was once engaged to when she was still a beat cop, cheated on her with her sister, Christina.
At the end of season seven, after rescuing her sister (Shattered), she discovers that she's now an aunt to Christina's baby daughter.
It is Scotty who first notices that there is something going on between ADA Jason Kite and her. Soon, romantic gestures flares between her and Kite, but it quickly ends in when Kite is put off by Lilly bringing her work home.
Scotty and Lilly were becoming good friends until her sister Christina arrives in town. An apparent beginning to making amends with her sister is also quickly cut short when Christina reveals that she has been dating Scotty, prompting Lilly to storm out after accusing Christina of destroying everything she touches. Despite their differences in the past, Lilly still tries to protect her sister when an NYPD detective, Mark Phillips, makes a visit to her apartment with an arrest warrant for credit card fraud. When Lilly calls Christina down to the office to meet her, Christina denies these allegations. Lilly doesn't believe her and tells her that she doesn't want any more 'early morning visits from the NYPD'.
A case where a social worker was murdered draws Lilly's attention more than most. She finds herself feeling a connection to the victim, Joseph Shaw. As she solves the case, she discovers that Joseph is still alive, his foster brother having been murdered and Joseph allowing the authorities and the murderers to believe it was him so as to avoid a second attempt on his life. He and Lilly form a relationship, having much in common, including their dedication to their careers and their love of cats. Lilly and Joseph are drawn apart, however, by the arrival in town of Lilly's former flame, Ray. Joseph breaks up with Lilly after finding out that she had gone to see him in the hospital after his rough lifestyle gets him beaten up. Sadly, it is at this point that Lilly had said a final good-bye to Ray. In a rare moment of total honesty, she blurts out to Joseph that she doesn't want to be alone.
Nearly two years after the end of her short-lived relationship with Joseph, Lilly meets her match in Detective Eddie Saccardo, a brash, egotistical fellow detective who works in the narcotics division. After working together on a few cases, Lilly comes to tolerate and even grow fond of Saccardo, especially after watching him use his personal history of drug abuse to get a witness to open up (although this could just be made up lie in order to seek the truth, there is no indication that Saccardo has even admitted to being a former drug user himself), a tactic Lilly often uses with witnesses in respect to her rough childhood. The two share a common trait of drinking, even in Jones Tavern. Sadly, their relationship came to a quick end after Saccardo was summoned to go undercover, he later reappeared in Season Seven's " Jurisprudence," after secretly texting Lilly. It's there when he gives her the gun in "Chinatown," but Lilly never uses it. They ended up going out for a night of fun and dinner, that's when Saccardo gives her a bracelet. The two then head to the beach, watching the sunrise together and share a kiss. In the end, Lilly discovers the apartment that he's been using is empty, and finds a box with a diamond bracelet inside. Saccardo is still undercover at this point. We later discover during the episode 7X18,"The Last Drive In," that Lilly is no longer with Saccardo.
One of Lilly's most challenging cases was that of a serial killer who stalked his victims like wild animals. One of the victim's bodies from 1985, Janet Lambert, is found by hikers in a wildlife preserve. The investigation quickly leads Lilly and the cold case squad to the grisly discovery of 8 more decapitated bodies. Soon, suspicion falls on George Marks, who gets brought into the interview room. During interrogation, he manages to get most of the team members agitated until Lilly is finally brought in to get George to confess to these gruesome murders. Instead, George hints about Lilly's dark secret to being the real reason why she became a cop. By this, he was referring to her attack as a child, which she rarely discussed.
Months later, when Lilly finally confronted George with evidence that he murdered not only the victims from their previous encounter, but his own mother as well, George forced Lilly into reliving her attack before their shootout which ended his life.
In 2007, shortly after the death of her mother Ellen Rush, Lilly interviewed a victim, Kim Jacobi, only to be ambushed by the person who had murdered Kim's family the year before. The entire homicide department was held hostage for a short time, with Detectives Scotty Valens and Kat Miller outside the building and under orders from the SWAT team not to break their perimeter. Scotty broke this order and approached Lilly's location in an observation room, where she was attempting to negotiate with the killer. Using a code word they had established that morning, Scotty shot the killer from the blind side of a two-way mirror in the interview room, but not before he shot Lilly in the chest. While undergoing surgery, Lilly apparently sees her late mother before her. She eventually recovers in time to convince John Stillman to reopen the Jack Raymes case, and credits Scotty for saving her life.
In May of 2009, while working on the Kate Butler Case, Lilly is involved in an accident which has her being pushed off a bridge while inside of her car, that plummetted forty feet into the river. However later on, Lilly is stalking Moe Kitchner, who is responsible for the accident. He is killed by the victim's father later on. She also rekindles a relationship with Eddie Saccardo, who is still undercover, but their relationship soon ends due to the fact that he's still undercover. Months later, the FBI is trying to recruit her, but we don't exactly know if that will indeed happen.