|“||I was going to wait until the "death do us part" bit, but it seemed a little on the nose.||”|
— Kai after killing Jo in I'll Wed You in the Golden Summertime
The antagonistic, twin brother-sister relationship between the witch Josette Parker and the hybrid, Kai Parker. Their relationship is severely strained due to Kai wanting to be the leader of Gemini Coven and killing most of their siblings, with the exception of their younger twin siblings, Liv and Luke. In the end, Kai killed Jo on her wedding day, and he died on the same day as well. Kai's death ended most of the Gemini Coven, which would have impacted Jo as well. Kai was eventually resurrected by the Devil after the events of What Are You?.
It was revealed in Fade Into You, that they were born in Portland, Oregon, in the 1970's, as a pair of twins for the Gemini Coven. Different from the rest of his family, Kai was born with no magical power of his own but was gifted with the ability to absorb the power of others. Based on the tradition of the coven, only a set of twins would become leaders, hence their name "Gemini".
On the other hand, at the age of 22, the twins have to merge power. The stronger one would absorb the power of the weaker, who would die as a result. Realizing Kai's disability, the Parker Family kept having children until another set of twins were born. Luke and Liv entered their lives as the 7th and 8th children of the family and the 2nd set of twins.
Kai killed 4 out of 7 of his siblings, attempted to murder the twins, but failed because Jo protected them. He refused to kill Jo right away, because she would be the one he would merge with to complete the ritual of becoming the leader of the Gemini Coven. Unexpectedly, Joshua Parker harnessed the power of a solar eclipse to trap Kai in a purgatory dimension for his crimes, which has kept him trapped in a snapshot of the day that he was imprisoned - May 10, 1994.
Throughout The Vampire Diaries SeriesFade Into You, Jo goes to Friendsgiving and is reunited with her younger siblings, Liv and Luke. She then reveals to the Mystic Falls Gang that Damon 's Kai is her twin brother who killed 4 of their brothers and sisters but was really targeting Liv and Luke. She says that since twins are in line to be leader, Kai wouldn't stand a chance unless Liv and Luke were alive. She continues talking about Kai and we learn more about the family's past through flashbacks. But since Joshua, their father, learn that there might be a chance for Kai to escape the Prison World, he attempts to kill Jo through a spell so that even if he did come out, he won't be able to merge with Jo. He fails.
Meanwhile in the prison world, Kai discovers that Jo left her magic in a knife in a tree stump. He stabs Bonnie and it is implied he returns to the living world.
In Christmas Through Your Eyes, Kai stabs a woman who lands up in the hospital, knowing that she will be helped by Jo. When the time comes for Jo to treat on the woman, Kai grabs her from behind and injects her with tranquillizer. Later, Kai is seen at the cemetery with Liv and Luke and Jo is chained up in a crypt. It is revealed that Kai wants to "reunite" Jo with her powers in the knife. He stabs her in the shoulder, claiming to be trying to give her back her magic. After a short confrontation with Luke, Kai kneels next to Jo and gets ready to stab her again. Jo stops him by telling him that she put her magic into the knife by choice, taking it back is also her choice, unless she bleeds to death. Kai then gives Liv the job to reunite her sister with her magic. Unsatisfied by the result, Kai tries to kill her younger sister but Jo stops him by claiming back her magic. When Damon and Alaric attack Kai, Jo stops them and tells them that he can't kill Kai as she does not think that it is fair to Luke and Liv to be forced to do the Merge. She also says that she can win against Kai in the Merge as she has regained her magic, she just needs to get stronger.
In Woke Up With a Monster, Jo is seen practicing magic in the Salvatore Boarding House with Liv's help and is having trouble with it. After Liv gets taken away by Luke, Damon is left with no option but to seek help from Jo in order to save Elena who is trapped in the high school with Kai. After Kai walks by without seeing them, they realize Jo's invisibility spell works. But the moment they find Elena, Jo's nose starts to bleed, but doesn't show it to Elena or Damon. Damon then notices Jo's nose is bleeding and realizes cloaking three people is too hard for her. Kai then sees them. She tries to run away but Kai catches up to her, grabbing her. But Jo is saved by Jeremy and Alaric as they shoot Kai with a crossbow.
In Prayer For the Dying, Kai is woken up from his induced coma by Damon who want him to save Liz by siphoning the magic out of her. He does half of the job and then disappears with Jo. They arrive at the park and start the merge, cutting their hands and chanting, until Luke interferes, knocking out Luke. After some convincement, Kai and Luke merge, despite a weak Jo's protests. Both brothers fall to the ground and Jo runs to Luke, cradling his head in her lap, telling him to wake up. However, Kai is the one to wake up, meaning he won the merge, leaving Jo grieving for Luke.
In The Day I Tried To Live, Kai goes to the Salvatore Boarding House, looking for Jo. He explains to Elena and Damon that he wrote a letter for Jo because he felt bad ruining her life. It is shown that, after the merge with Luke, Kai has stolen Luke's ability to do permanent magic but also part of his humanity.
In Let Her Go, Jo is seen vomiting in the bathroom until there's a knock on the door. It is Kai who claims that he wasn't feeling so good and ends up coughing up blood. He pleads her to help him, since if he dies, the entire Gemini Coven, along with her, dies with him. At the hospital, Jo checks Kai's vitals and responds that everything appears to me normal and then comes to a conclusion that it is something magical making him sick. As Kai starts to cough up more blood, he grabs Jo's arm, beginning to absorb her magic. Alaric quickly intervenes and disconnects Kai from Jo. Kai claims that it made him feel a lot better and that Jo needed to give her magic to him. Jo replies that it will kill her. After consulting Alaric, Jo decides to give the rest of her magic to Kai. When the spell is done, Kai thanks her again and whispers that she is pregnant and walks out.
In I Never Could Love Like That, Jo learns from Elena that Kai is trapped again in a prison world. She is seen overjoyed and says that they must destroy the Ascendant as soon as possible. At the end of the episode, Jo reveals that the people trapped in the prison world with Kai are no ordinary vampires. They were siphoners, turned into vampires but didn't lose their powers. But with the ability to use magic and the bloodlust, they are described as extremely deadly. Jo wants to destroy the ascendant because if Kai does get turned into a vampire, he mustn't be allowed to escape. He will be unstoppable.
In I'd Leave My Happy Home For You, Jo learns from Lily that she is carrying unborn twins. Scared for the future of her children, she shows an ultrasound of the babies to Alaric and explained that if Kai ever finds out, he will come after them since the twins are the only one who could possibly stop Kai. Together, they decide to leave right after getting married and hide from the coven so no one will get a hold of the twins.
In I'll Wed You in the Golden Summertime, the twins are seen together at the end of the episode. As Jo starts to recite her vows to Alaric, she stops mid-sentence, her eyes widening. As she looks down at her stomach, blood is starting to spread on her wedding dress. After being stabbed multiple times, she lets out a scream and collapses into Alaric’s arms. That’s when Kai turns off his invisibility spell. Armed with a knife covered with his sister’s blood, Kai smiles before turning his attention to the wedding’s attendants. He then inflicts massive aneurysm to all and destroys most of the church's windows and fixtures by producing high frequency sound waves.
In I'm Thinking Of You All The While, it is confirmed that Jo was killed by Kai, effectively ending his family relationship with her.
- In a deleted scene, Kai (post-Merge) revealed he was jealous of Jo being able to live a normal life away from their coven while he was imprisoned for being the "family reject". She was moved enough to accept giving her powers to him if he stayed out of her life for good.