Kirsty Branning is the secret wife of Max Branning. On Christmas Day 2012, she is revealed by Derek Branning that Max is having a affair and hiding it from his wife Tanya Branning. She then knocks on Max and Tanya's door minutes before they're supposed to get married. Tanya is angry after they kiss on 4 January 2013, and so leaves the square. On 22 January 2013, Max demands she sign divorce papers, but Kirsty refuses. But after she is told by Kat Moon that she is making Max stressed, she signs the divorce papers and leaves Walford in a taxi. However, on 4 February 2013, Max is found sleeping with her when Max's daughter Abi Branning finds them in Room 14 at the B&B. She stays at Walford and gets her job at The Vic back on 11 February 2013.
Max and Kirsty eventually buy a flat on the Square, which Tanya isn't very pleased about when she returns. During an argument, Max says that he will always have a special connection with Tanya because they have children, so Kirsty blurts out that she is pregnant. She tells him she plans for a termination, but after Max talks to Tanya, he doesn't want her to. Kirsty, however, tells Kat that she isn't really pregnant. Max books an ultrasound scan for Kirsty, but she secretly cancels it without telling Max. When Max finds out, he storms into the hospital and shouts at the receptionist. Kirsty then suffers a panic attack, fearing that she may get found out. Max then apologises for pushing Kirsty into having the scan. Kirsty's ex-boyfriend Carl White arrives on the Square in June 2013, and stirs up trouble for Max and Kirsty's relationship. Max's eldest daughter Lauren Branning's alcoholism reaches a dramatic climax when she collapses and is rushed to hospital and is diagnosed with hepatitis. While Max and Tanya are vigils at Lauren's bedside, Kirsty takes a pregnancy test, hoping she may actually be pregnant. Unfortunately, Carl spies Kirsty talking to Kat about the situation, which leaves Kirsty reeling whether he heard any of their conversation.
Kirsty takes the test which reveals that she is not pregnant. She hides it in the rubbish bin, hoping that Max will not find it. When Max returns home from the hospital, he and Kirsty argue, leading to the rubbish bin to fall over. As soon as Kirsty sees the pregnancy test, she grabs it and hides it behind her back, but Max notices this and a physical struggle ensues over the test. Kirsty eventually drops the test and Max finds out the awful truth about her fake pregnancy. Max then heads straight over to Tanya's house, but Kirsty follows him, leading to a public row in the street. Kirsty then gets drunk to hide the pain of the split. When Tanya flees Walford with Lauren and baby Oscar, Max decides to take Kirsty back, much to her delight.
On 15 August 2013, Kirsty is attacked by Carl's younger brother Adam. He attempts to mug her, but Carl comes to her aid and Kirsty runs off, and Carl attacks Adam. Later that day, Kirsty tells Max about the attack, and when he insults her and Carl, she slaps him. Max then calls the police and Carl is arrested for dealing drugs, much to Kirsty's shock. No drugs are found on Carl, so Kirsty takes him into the B&B and it seems that Carl begins suffering a panic attack. Max then storms in and tries to attack Carl, but Kirsty manages to stop him. In early September 2013, Max becomes increasingly riled by Carl's presence on Albert Square, and the two come to blows in the middle of the street, but the fight is broken up by Max's younger brother Jack Branning. The next day, Carl invites Phil Mitchell for a ride in his car for work, but Carl drives wrecklessly and ends up crashing in the car, leading to Phil flying through the windscreen. Carl escapes the accident with minor injuries, but Phil is air-lifted to hospital. The police then arrive at The Queen Vic and arrest Max for attempted murder, after a "witness" saw him tampering with Carl's car breaks. The "witness" is Carl's blackmail victim, Ian Beale. Max is released from prison, but when Carl winds him up again, Max punches him across the face, leading to him getting arrested yet again.
Kirsty, Jack and the rest of the Brannings learn that Ian is the "witness", which leads to Jack attacking Ian and a fight ensuing as Kirsty backs Jack up. To help Abi, Lauren and their grandmother Cora Cross, who are struggling with money, Kirsty sleeps with Carl and steals money from him the following morning. Max is released from prison following a court hearing in November 2013, but he kidnaps Carl with the help of Phil. Max leaves and leaves Phil to dispose of Carl. Max learns that Kirsty and Carl had sex while he was in prison and breaks up with her, leaving Kirsty devastated. Kirsty tries to make amends with Max on New Year's Eve, but Max is having none of it.
In January 2014, Carl is nowhere to be seen, and everybody is unaware that Ronnie Mitchell murdered him. She asks around the Square, but nobody knows where Carl is. When Max learns that Tanya is now in a new relationship, he takes Kirsty back, which delights her. She makes a big mistake, however, by making the dinner with wine, knowing that Lauren is an alcoholic. Kirsty is subsequently thrown out of the Branning household. The next day, Kirsty visits Max asking for money, but he only gives her £20, which infuriates her. She then seeks advise from Shirley Carter, which gives Kirsty a villainous idea. At Ian and Denise Fox's engagement party, Kirsty pretends to be drunk and tries to hug Max, sneakily taking his car keys in the process. Everybody in The Queen Vic then hears a car horn beeping, and Max, Lauren and many other locals walk outside to see Kirsty in Max's car. Kirsty throws divorce papers at Max andd his house keys. She then throws her wedding ring behind her as she leaves Walford for good.