This is the relationship between a vampire Caroline Forbes and an Original Rebekah. Originally Caroline hated and feared Rebekah because she was Klaus' sister (making her an Original), and saw that they worked together. But despite, or possibly because, they are very much alike in personality, Caroline and Rebekah often clash, even over trivial things.
In The Reckoning, Rebekah confronts both Tyler and Caroline, then taking Tyler to Klaus, leaving Caroline unconscious. Later on, Rebekah holds Caroline back, while Klaus gives Tyler Elena's blood to drink, thus completing his transition into a hybrid. Caroline seems upset and concerned about Tyler becoming a hybrid.
In Smells Like Teen Spirit, Rebekah starts to look after Tyler (on Klaus's demand) by getting him people to feed on. When Caroline finds out about this, she seems incredibly upset. Later on, Tyler praises Rebekah's stunt and makes Caroline feel jealous. After this Caroline begins a mutual dislike for Rebekah.
In Heart of Darkness, Rebekah organizes the Decades Dance, much to Caroline's anger. They have quite a small argument, with Matt Donovan siding with Rebekah. After, it was revealed that Matt only did it so that Rebekah couldn't spoil the time between Tyler and Caroline, after Tyler had returned.
In Before Sunset, Rebekah is cleaning up after the 20's Decade Dance, which was the night before. Caroline comes and starts to help her, and she apologizes to Rebekah after what had happened to her mother, with Rebekah apologizing to Caroline, about Alaric Saltzman. Rebekah leaves and says she'll start on the gym, but gets stopped by Dark Alaric, who she believes is dead. She goes to attack him, but he is stronger and is about to kill her with the White Oak Stake, until Caroline comes and saves her. They both stab Alaric, revealing that he hasn't died. Caroline and Rebekah disappear, with Caroline running towards her car. Alaric shows up and snaps her neck, dragging her away whilst Rebekah watches from a safe distance.
In Growing Pains, both of them were taken together by the council members, to the cell. Klaus shows up in Tyler's body, to save Caroline, leaving Rebekah all alone.
In She's Come Undone, Matt and Caroline are talking about graduation at the Salvatore Boarding House , and Rebekah arrives. Rebekah says she come to make a peace offering with them, and orders them burgers. Caroline says it was nice of her to drop by and is about to say that she should go, but Rebekah says she's going to stay and help Matt study. Caroline gets jealous and asks him why he didn't come to her.
In The Walking Dead, Rebekah had found Caroline that has been compelled by Silas. Matt and Rebekah tried to get her to stop but she wouldn't listen. Matt called Damon and gave him the update on Silas and Caroline. Caroline was still determined to cut off her wrists -- until Rebekah smacked her back to her senses.
Caroline and Rebekah have their differences, but prove to have empathy between them.
Even if they don't like each others, maybe even enemies. They are two girls who are currently the most close to Stefan. Caroline is Stefan's best friend and Rebekah is Stefan's ex-girlfriend and close friend.