The relationship between the former vampire/Traveler Witch doppelgänger Katherine Pierce and the vampire Isobel Flemming. Their relationship was both malicious and friendly. They were partners in crime. Their current friendship-status is unknown since Isobel was compelled to turn Katherine in and Isobel killed herself before they could reconcile.
Throughout The Vampire Diaries Series
Katherine and Isobel were working together, along with John Gilbert.
Katherine and Isobel were apparently friends as shown in Season Two. Katherine was living at the Salvatore Mansion when she heard from Damon and Stefan that Isobel had come back to town. At her foreclosure, Isobel arrives and says, "Merci mon chéri." With a wine bottle in her hands. Isobel then feels a presence behind her and uses her vampire speed and strength and pushes the figure (Katherine) on the neck to the wall closest. Katherine smacks Isobel's hand off her neck, grabs her neck and pushes Isobel to the other wall and says, "Nice house. Isobel answers by saying, "Nicest foreclosure in town."
Both women smile and Isobel tells her to come here and and they hug. Katherine comments that she hears Isobel's been busy and Isobel answers by saying that she's making a deal with Klaus to save her life. Katherine and Isobel sit down and drink blood. Katherine asks Isobel what was up with the surprise visit at the Gilbert Mansion the night before. Isobel answers, "John told me Ric was dating Auntie Vanilla. I got jealous." Katherine wants to cut to the chase and asks Isobel to let her what she knows. Isobel answers by saying, "I couldn't get anywhere near Klaus. But I found my way to someone in his trusted circle . . . . One of his witches." "Ugh. Klaus and his witches," Katherine says.Isobel continues, "He said that Klaus is willing to grant you your freedom. If we deliver the moonstone and the doppelgänger." Katherine says that she can get the moonstone and Isobel asks her if she's prepared to betray the Salvatores again. Katherine answers, "I was more than willing to play it their way if I had to . . . but they're floundering. Their witch has lost her powers, they've used their only weapon to kill Elijah. If I stick with them I'm dead. You showing up changes everything."
They both smile and drink their wine-scented blood. Katherine later gets the moonstone and goes back to the foreclosure to meet up with Isobel but finds that she isn't home and calls her.Isobel picks up, "Are we good to go?" Katherine says, "I'm at your house. We have to hurry. Damon knows your lodging tricks. It will take them 20 minutes to find out where you're staying." We'll be long gone before that," Isobel answers. Katherine: "Good. How far are you?" Isobel: "I'm sorry, Katherine . . . but I had to do what I was told. He wanted the moonstone and he wanted you." Katherine: "He? He who?" Isobel hangs up and Katherine realizes. She turns and sees that Maddox is there.
Katherine shows her fangs and attacks, but Maddox is a stronger witch than she expected and takes her down.
Katherine comes to in Alaric's apartment to find that Maddox has transferred Klaus' essence into Alaric's body.
- Both Isobel and Katherine are distantly related to each other by blood.
- Isobel bore Elena Gilbert, Katherine's doppelgänger.
- Both Isobel and Katherine slept with and dated Damon Salvatore.
- Katherine also sired Damon as a vampire, but Damon also sired Isobel decades later.
- In the end, Isobel betrayed Katherine due to Klaus Mikaelson's compulsion. They never made up being Isobel killed herself by burning in the sunlight shortly after betraying Katherine to Klaus.