Series 3
Ih the opening episode "Everyone", her hostile behaviour towards Emily is made clear as she tries to shed her twin, establishing her individuality, and seemingly trampling on her sister as she does so. She does it to the point where Effy comments that Katie treats Emily like a "doormat". She wants to be popular and tries to befriend Effy Stonem, who seems to be the most popular girl in school. Katie is deeply homophobic against Naomi, as she thinks that Naomi forced herself on Emily.
In "Pandora", during Pandora's birthday party, Katie sees Naomi and Emily kissing in a jump house, and is shaken. Soon, her boyfriend, Danny, and all his friends, show up and crash Pandora's party, and turn it into a mess. In "JJ", she is revealed to be in a sexual relationship with Freddie after dumping Danny. After Emily and JJ catch her having sex with Freddie, JJ accidentally reveals Emily's homosexuality to her, and he later invites Emily to stay the night at his house as she is too afraid to go home and face Katie.
In "Effy", the group (minus Cook) have a party in the woods, where Katie and Effy's competitive nature regarding their 'queen bee' status is made clear. After Cook crashes the party and reveals that Effy doesn't want him anymore, Katie grows more anxious that Effy instead wants Freddie, leading to her heading off in the woods alone. Effy starts to suffer from a bad trip, having taken mushrooms. She meets Katie again; eventually, the two come to blows, with Katie pinning Effy down and spitting on her before attempting to choke her, prompting Effy to pick up a rock and strike Katie in self-defense. Effy returns to the camp and has sex with Freddie, but the next morning when people question Katie's whereabouts she says nothing, ashamed and unsure of what to do. Eventually Effy calls an unknown person (presumably the police or an ambulance) and requests they come and get Katie.
In "Katie and Emily", Katie's injuries as a result of her fight with Effy make her too embarrassed to leave the house, so Emily disguises herself as her sister and sits for her exam. Katie ignores Emily when her sister comes out to her, telling her "you're not gay, you're just stupid". She later confronts Naomi after Emily comes out to her parents, telling her to stay away from Emily. Freddie reveals to her that JJ had sex with Emily, and she guilt-trips Freddie to going to the coolege ball with her, and sets up Emily and JJ. Katie intercepts Naomi's phone call to Emily and tricks Naomi into meeting with her, where she reveals Emily's affair with JJ and warns Naomi not to come to the ball.
Eventually, Naomi shows up at the ball and Katie pulls her aside and reminds her to stop talking to Emily. However, Emily overhears what Katie did and they both begin to fight. Emily gains the upper hand but doesn't hurt her sister, instead picking her up and saying that she loves Katie, but she isn't her; Emily is gay and is in love with Naomi. Katie finally seems to accept that her sister is her own person.
Series 4
In "Thomas", Katie is seen at a huge party at the club that Thomas works; she is present when Sophia commits suicide after taking MDMA. In "Emily", her new boyfriend, Sam is revealed. Later, Emily, unable to understand why their mother can't accept her homosexuality and her relationship with Naomi, decides to move out, leaving Katie heartbroken. It is revealed that their father had lost his gym, making him bankrupt.
Her central episode develops her character the most. She is working with her mother on her new wedding planning business, and their first client is Brandy - a stereotypical Bristol Rovers WAG and her bad-mannered mother Vivian (Jenny Eclair). Katie, realising that she hasn't had a menstrual cycle for some time, goes to the doctor to see if she is pregnant. The doctor tells her that she isn't pregnant, but going through a premature menopause; thus she won't be able to have children. A shocked Katie leaves the clinic and dumps Sam. After trying to reach Emily, she heads home to tell her mother. Before she's given the chance to talk about her problem, her mother makes a discovery whilst on the phone to the bank - the Fitch family are broke and have lost everything, with her father Rob hiding the evidence under the couch, resulting in a hostile argument between the parents. When Katie gets to Brandy’s hen party, the theme was changed without her knowing, resulting in her wearing an unfitting 'sluts and suds' outfit as opposed to the Bristol Rovers outfit theme.
Her ex-boyfriend Danny's new girlfriend is present at the party, as well as Effy and Freddie, and Katie rejects Effy's invitation to join her. Danny's girlfriend then humiliates her and she retaliates by striking her in the face while yelling "I'm Katie fucking Fitch, who the fuck are you?", resulting in her and her mother being sacked. Outside, Effy and Katie bond and appear to strike up a friendship. Katie heads home and the bailiffs arrive to repossess their home and they're left with one option - do a runner with as much stuff as they can, being forced to move to Naomi's house, where Emily is living.
Katie realises things are not well with Naomi and Emily when they move in. Emily throws an impromptu BBQ with revellers from the night before, and displays some unstable traits. When Emily's request for more pills is declined, she grows angrier and out of spite, kisses a girl in front of everyone. Katie watches on in shock as Emily runs at Naomi and pushes her into the paddling pool. Fed up with hiding and secrets, Naomi announces to everyone that she sold Sophia the drugs and also had slept with her.
Katie decides to take a bath, and Thomas accidentally locks himself in the room, not knowing Katie is in there. Katie wants to make sure she's still attractive, and they start to kiss. Katie asks him if he thinks she is a bitch, because she feels like many people see her that way. However, Thomas tells her that she is honest, even if it hurts people to hear it. This event causes Katie to feel better about herself, and she decides to help reunite Thomas and Pandora to make him happy too. The Fitch family return to their old home to collect Jenna, and Katie tells her she can't have children and that she wants her mother to love her no matter what. She tells her mother to stop pushing them away; her mother takes Katie's advice thus reuniting the family.
In "Effy", Katie is dancing in the Judgment Day Parade, and sees a freaked out Effy (who is falling deeper into her psychotic depression) and pulls her out from the crowd. She signals to Freddie to get her, as Effy was visibly shaken. The two then take her to Freddie's grandad's nursing home and he suggests Katie take her to the cafeteria to get some tea. As Freddie is talking to his grandad, Katie bursts into the room and scaredly tells him that Effy won't come out of the toilet. Freddie races there and is quick enough to save Effy who has just cut her wrists.
Katie then seems to spend the majority of her time at the psychiatric hospital Effy is being kept in following her suicide attempt. In the finale "Everyone", she and Pandora cheer her up with a song Pandora has written. On the bus heading home afterwards, Katie starts her plan to help Thomas and Pandora, so drops hints about Thomas tutoring her one-on-one for French, hoping to spark up some jealousy; it doesn't seem to work though. They both know Pandora is outside but what they're not aware of is that she took various exams in secret and got an A* on her French exam so can understand everything. After she has run away, Katie asks just what she has actually been saying to which Thomas gestures towards her chest. As they are play-fighting, Katie kisses Thomas again before pulling away as she knows that he loves Pandora.
Katie received two Cs and a B from her A-levels. Arriving with Emily at Freddie's shed, Katie witnesses Naomi and Emily finally make up properly, and in her last scenes she is drinking shots with everyone and having fun.
