|“||We're gonna have so much fun together.||”|
— Katherine to Caroline in Bad Moon Rising
The relationship between Caroline Forbes and Katherine Pierce started in The Return when Katherine turned Caroline into a Vampire. This relationship was based on Caroline's fear of what Katherine would do to her and Matt if she didn't obey her orders.
Katherine and Caroline had no interaction in season one because Katherine wasn't in town until Founder's Day.
At the end of The Return, late in the night Katherine entered her hospital room. Caroline mistook her for Elena but Katherine corrected her. She asked Caroline to give a message to the Salvatore Brothers. She picked up Caroline's pillow and smothered her to death. It is certain that she knew that Damon's blood was still in her system when she killed Caroline and therefore would return as a vampire to give the brothers Katherine's message, "Game on". She then walked out of the hospital.
In Brave New World, at the carnival, Caroline gave Damon, Katherine’s message letting him know that she was a vampire by telling him that she was remembering all of the times he made her forget things. She pushed him down sending him sliding across the floor with her new found super-strength to emphasize the point. After Caroline killed Carter, Stefan and Elena turned up and Caroline mistook Elena for Katherine at first but she explained, although Caroline still didn't understand why she looked like her and why Katherine did this to her. When Stefan took Caroline to the bathroom to clean up she asked him why Katherine turned her. Stefan shook his head and replied, "I don't know. I wish I did." He promised that he wouldn't let anything happen to her and embraced her.
In Bad Moon Rising, the next morning Caroline is in her bed sleeping. She awakens to find a woman beside her bed, again, and recoils. "Elena?" she asks. "Nope. Try again.". "Katherine." Caroline concludes correctly and recoils further against the headboard of her bed as Katherine sits on it. "Don't be frightened." "We're gonna to have so much fun together." She says deviously.
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 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 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 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 "ask" 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 says to her don't lie to her 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, "Good bye Katherine."