|“||I'm not worried about you offing me. Because we both know I made you better by making you a vampire.||”|
— Katherine's last words to Caroline in Gone Girl
Katherine turned Caroline into a vampire, a short while after the latter got hospitalized, and about few hours after Katherine returned to Mystic Falls. Originally, Katherine planned to hand her over to Klaus to use her as the vampire in the sacrifice to break the Curse. Katherine later threatened to kill Matt, thus using Caroline as her spy—to get Elena and Stefan to break up. Later, Caroline confess to Elena about Katherine threatening her, and Elena comforts her. Caroline gets her revenge in helping imprison Katherine in the tomb, as the Salvatores let her in on their plan—as she now a vampire. After that, when the news of Klaus and the curse started reaching Mystic Falls (through Rose and Trevor), Elena—desperate for some answers—turn to Katherine, and convinced Caroline to lift the rock that blocked the entrance to the vampire tomb. From there, the two vampires exchange some bitter looks and some harsh words—all from Kathrine's side though.
Katherine and Caroline didn't interact with each other until Homecoming, because after Klaus broke his curse, Katherine immediately fled Mystic Falls. That night, at the Homecoming dance, Katherine once again pretended to be Elena, to ensure the latter safety from Mikael, and to be part of the plan to kill Klaus: she told Matt that she was Elena, and she needed him as a back up escort. At Tyler's house, they met Caroline, who told them that Klaus was inside the house, but Katherine told Matt to act normal as Caroline didn't know about the plan (and even though Matt didn't know the whole plan either, and didn't know he was talking to Katherine at the time—he agreed), but the plan failed very quickly, and Katherine "ran for the hills like usual" - as Damon put it.
After Klaus left Mystic Falls, Katherine returned there, but she and Caroline never met, duo to Silas' mind games, and Bonnie's plan to kill him using Katherine.
Katherine and Caroline did meet again when Katherine used the dorm showers in Caroline's room, but Caroline quickly realized that she wasn't a vampire, and not Elena. Katherine, having no other place to go, strikes a deal with Caroline: she will help Caroline with the anti-vampire society—and in exchange, Caroline let her live in the dorm and use Elena's meal card. From there on, the two once enemies, turn to some kind of allies, due to Caroline's empathy towards Katherine, and her poor state. After Katherine's death, Caroline is the only one who feel kind of sad for her.
Throughout The Vampire Diaries Series
In The Return, Katherine visits Caroline in hospital where Caroline mistakes her for Elena. Katherine corrects her and says to tell Stefan and Damon, "Game on." She then smothers her to death with her pillow.
The next day, Caroline confronts Damon, giving him Katherine's message. Caroline later mistakes Elena for Katherine and is terrified but Elena explains it to her. She asks Stefan why Katherine would do this to her and he says he doesn't know.In Bad Moon Rising, In the morning, Caroline's asleep and wakes up to find Katherine by her bed. She mistakes her for Elena but Katherine corrects her again. Caroline's scared, but Katherine tells her not to be.
In Memory Lane, Caroline tries to persuade Elena to break up with Stefan, following Katherine's orders. After the car breaks down, Caroline doesn't want to be left alone presumably because Katherine will get her. Later at the Mystic Grill, Caroline is in the Grill's lavatory at the sink freshening up in the mirror. Then she hears footsteps and Katherine appears in the reflection. Caroline turns around with trepidation. Katherine is angry because Caroline didn't do exactly what she said and occupy Elena and keep her from returning home and contacting Stefan (Elena came home just as Katherine "escaped" from Stefan and menace Elena). Caroline told her that she did all she could to convince Elena that her relationship with Stefan was doomed. As Katherine menacingly stepped forward Caroline said she thought she got to her. "I hope so, because let's not forget. I've already killed you once; I can easily do it again." Katherine said with undisguised contempt. When Stefan and Elena break up, Caroline listens to them probably then reports it to Katherine.
In Kill or Be Killed, Elizabeth Forbes notes that Elena was there the previous night. Reflexively Caroline corrects her saying that Elena wasn't there but then she catches herself saying that Elena was there and needed to talk. She had realized her mother was unknowingly referring to Katherine. After they brought Caroline's mom to the Boarding House, a crying Caroline tells her that she can't go home. Elena asks why. Tenderly Elena coaches it out of her: it is because Katherine is going to be there. She confesses that she will want to know everything that happened that day and also that she had to spy on Elena and report back to Katherine. Elena tells her that she knows and she was so mad at Caroline then she tried to put herself in her position to try to figure out why she would do that to her and Stefan after being such a friend to her. Elena gets up and sits next to her friend. Insightfully she asks "Who did she threaten?" "Matt" Caroline said Saying that she was very scared of Katherine. Elena said she should be, that they all should be scared of her. Caroline ask why is Katherine doing this to them. "That's the million dollar question" Elena says. Caroline, despite being many times stronger than Elena settles in Elena's embrace with her head against her chest feeling safe and falling asleep in the arms of her best friend.A few days later in Masquerade, Caroline is at the Salvatore Boarding House relating a story to Damon and Stefan, about her encounter with Katherine at the Mystic Grill. She had gone to the to Grill "gawk" and "stalk" Matt. After an awkward moment he asked her if she wanted a table. She told him in a giggly manner that she just wanted to use the little girl's room. Damon, impatient tells her to get to the point. Caroline then tells them that when she went to the lavatory (to follow up on her "dufus" lie to Matt) she met "Elena" there but she really instantly knew it was Katherine possibly from the lavender dress that was unlike Elena would wear casually and the wavy hair that distinguishes the otherwise completely identical beings. Katherine pretended to be Elena asking Caroline in her reserved way if she was okay. Caroline made a bit of small talk pretending to be sad then using her Super speed bolted for the door to rush by Katherine, but Katherine, either by being faster or anticipating the attempt catches blocked the door. "What gave me away? Was it my hair or was it my clothes?" She rhetorically asked Caroline. She told Katherine that she knew Elena was at home. Katherine told her she needs her to deliver a message to Damon and Stefan: "Tell Damon and Stefan I want the moonstone or I will rip this town apart until it rains blood. Tonight. At the Masquerade Ball." Stefan says that she wants to get the stone in public. the death of Mason threw her off guard. Damon says that she is running scared and what she did to Jenna was an act of desperation. Stefan counters that they must not underestimate her. Caroline just wants to give her the moonstone so she would leave. Later at the Ball, Caroline's phone vibrates and she sees a text message from Jeremy saying "your turn". Seconds later she is walking in a lonely isolated part of the mansion away from the party. She walks toward a room and she reaches for the handle but a small fist reaches out and catches Caroline's wrist. In a flash Caroline is pinned against a wall face first with her arm twisted behind her back and grimacing in pain with Katherine's other hand against her head. "What are Stefan and Damon up to?" Katherine "asks" Caroline without preamble. What do you mean?" Caroline ask with her lips almost pressing against the wall. Katherine says that Jeremy is trying to lure her to the lake. "What is going on?" She demands. Caroline plays dumb saying she doesn't know. Katherine spins her around and grabs her by the neck again pinning her against the wall and tells her not to lie to her because she knows they are up to something. Caroline then leads Katherine into a room as Katherine walks in Caroline smiles saying, "I did it, I didn't think I could trick you but I did it." Leaving an angry Katherine in the spelled room as Caroline walks away she says, "Goodbye, Katherine." In Katerina, after hearing the news about Klaus from Rose, Trevor, and the Salvatores - Elena, desperate for some answers, turn to Katherine, and convinced Caroline to lift the rock that blocked the entrance to the vampire tomb, there the two vampires exchange some bitter looks and some harsh words - all from Kathrine side though. After that, Elena told Caroline that its fine for her to leave as Katherine can't hurt her - due to the spell on the tumb, Caroline agrees, and leaving to distract Stefan, so he won't know that Elena went to talk to Katherine, as Elena asked her.
In Homecoming, at the Homecoming dance, Katherine once again pretended to be Elena (at Damon's request), to ensure the latter safety from Mikael, and to be part of the plan to kill Klaus: she told Matt that she was Elena, and she needed him as a back up escort. At Tyler's house (The school gym was flooded) they met Caroline, who told them that Klaus was inside the house, but Katherine told Matt to act normal as Caroline didn't know about the plan (and even though Matt didn't know the whole plan either, and didn't know he was talking to Katherine at the time - he agreed) - but the plan failed very quickly, and Kathrine fled - and Katherine and Caroline didn't meet until Katherine was human again - as Elena shoved the cure down her throat after Graduation - in season 5.
Handle with Care, Caroline enters to her room in the college and meets with who she thinks is Elena fresh
out of the shower, she says to her that she Don't want to know if Tyler called, and they have to finish packing her things to leave college for the threats of professor Maxfield, Caroline realizes that Elena has no idea what she's talking about, then she takes her neck because she knows that it is actually Katherine and not Elena. Katherine says that her need a place to live, Caroline at first says no but Katherine tells her that she can help with the professor Maxfield and Caroline accepts.
Later, the professor Maxfield in his laboratory, is conducting experiments with vampire blood, Caroline appears and Katherine takes the opportunity to inject a substance into his neck, the professor faints in the floor. Caroline and Katherine planned drain the vervain in his system to compel him to tell them everything he know about vampires, Katherine takes over and begins to drain blood from the professor, he tells them that the secret society planned to invite Elena to belong the group but for his suspicions that she was a vampire, they desisted, Katherine then takes the decision to impersonate Elena and Caroline stays with the professor.In Death and the Maiden, Caroline is at Whitmore College and she meets Nadia, they are talking and then comes Katherine, Nadia tries to tell Caroline that Katherine is her mother, but Katherine interrupts her and doesn't permit, Caroline leaves and leave them alone.
When Katherine is trying to clean her blood after being cut by Qetsiyah in her hand,Caroline offers her blood to heal her but Katherine can't drink vampire blood.
Later in the college Caroline is preparing her room for Bonnie too, then Katherine arrives and understands that she has to go. When Nadia comes and starts talking to Katherine, Caroline discovers that Nadia is the daughter of Katherine and she is surprised about that.The Cell, Katherine calls to Caroline for help since she overheard some of Stefan's PTSD, Caroline arrives with the safe where Stefan was locked up for 3 months, she has the idea of putting him back in his box.
Later, Caroline is reading a book in search help but wasn't working very well, Stefan was still freaking out and Katherine was questioning whether or not Caroline and Stefan had ever hooked up, Caroline responds that they are just friends and she don't want to continue talking about it. Katherine had the idea of shutting herself in the box with him to help, meantime Caroline watched them. When Katherine and Stefan are getting closer and seemed like they were going to kiss, Caroline felt the need to open the box because she thought it was too quiet .
At the Salvatore's house, Caroline is trying to move the safe all by herself. She yells for Stefan to help but he's cleaning up the room he tore up earlier. After Caroline heard when Stefan and Katherine are kissing and she leaves a message to Elena to call immediately.In The Devil Inside, As Matt throws welcome party to Tyler she is there and meets Katherine in Elena's body. She calls Damon to come to the party so he could fix thing with Elena. She thinks that this is Elena and tells her that she did a bad thing and confesses to her affair with Klaus. Katherine pretends to care so that she would not understand that Elena is gone, but it is annoying her. When Katherine sees Tyler standing behind Caroline she takes opportunity to get rid off her and makes sure Tyler knows that Caroline slept with Klaus. Then Katherine leave as Caroline is distracted with Tylers reaction.
In Total Eclipse of the Heart, Caroline, Bonnie and Katherine who is still posing as Elena, attend college classes, they walks into a classroom and Caroline is talking about a fresh start in her life as single. Caroline decides to move on from her relationships with Tyler and Klaus and asks Bonnie and Elena to attend at Bitter Ball with her. she tells Bonnie to assist without the company of Jeremy what Bonnie accepts, the three agree to attend the dance.
When Katherine asks her advice about earrings to wear at the party, she tells her that she doesn't have to pretend with her because she know that her is on edge because of the whole Tyler-Klaus thing and that she knows that she must be sad over her break up with Damon and that she don't have to come to the party if she doesn't want to. At that time, Katherine gets a call from Stefan and decides to invite him, Caroline looks at her curiously.At the Bitter Ball, Katherine and Caroline come to dance, then Stefan appears and Katherine invites him to dance. They are dancing and talking but Caroline interrupts them and Caroline tells him that Katherine promised to join her at the shredding station, Stefan is agrees, so she and Katherine leave him alone. When they arrive at the shredding station, she pulls Katherine aside and destroys pictures of herself with Tyler and also the picture that Klaus drawing for her, she says that she is making a decision and that these relationships are over. She advises Katherine to slow down her relationship with Stefan, because she thinks that perhaps she is just feeling bored or nostalgic and lonely and she will do the same,when Katherine seems bothered, Caroline tells her to forget everything she said, Katherine agrees and leaves. Katherine beguns to worry that Caroline might have feelings for him, she say Nadia that ask to Matt if Caroline has feelings for Stefan.
Later, Stefan and Katherine are embrace in the room of the college, Caroline arrives and interrupt them.In No Exit, Caroline and Stefan are talking when Caroline arrives, she is still in posession of Elena's body, she offers to accompany Stefan to find Damon, Caroline is surprised to see her but Katherine tells her that Stefan is helping in her studies. At that moment Caroline's phone rings and is Tyler who is worried about Matt. Katherine is standing behind Caroline, listening to the conversation overhears him, because Caroline and Tyler are talking about Matt who is is kidnapped by Nadia, when she heard that Caroline will go to Tyler's house, Katherine tells Caroline to go with her but Caroline disagrees telling her that she will deal with Damon. Katherine says that she will going to get some clothes and left Caroline and Stefan alone.
In Gone Girl, Caroline and the others begin to looking for a solution on how to draw Katherine to them and so to stab her with the traveller knife. Caroline says that Katherine going to see a sneak attack from a mile away and that they've got to get her to come to them, get her guard down, corner her, Caroline also says that they've got to invite her to something that Elena can't say no to.