Katherine Von Swartzschild's History
Saved by The Old One
Born in Germany during the 1400s to the Baron von Swartzschild, Katherine was extremely weak and frail during her childhood and her last illness was unfortunately terminal. She recalls later that she was unable to breathe and too weak to move. Her father was devastated. Gudren, her maid, sought out a vampire named Klaus who was in the local village at the time. Klaus made her into a vampire, saving her from death and making her much stronger than she had been in life. It is Gudren who advises her on the practicalities of being a vampire, telling her to have a lapis lazuli ring made as a talisman against the sun, and bringing her small animals for feeding. Sometime after this her father brings her to Italy during the summer, as he believes the warmer climate will help her recover. He is likely an old and good friend of the Conte Di Giuseppe Salvatore, as he stays with him and leaves Katherine at the estate when he has business in Florence.
The Brothers' Wrath
It is here that she meets the Conte's handsome, youngest son, Stefan Salvatore, and the two begin a romantic relationship and courtship; she shares her secret of her being a vampire with him. Katherine and Stefan are deeply drawn to each other and enjoy being in each other's presence. They spend a lot of time with each other every day. They fall deeply for each other enough that their fathers begin speaking of marriage. During her stay at the Salvatore Estate, it is noted that she only leaves her chamber at twilight, but Conte Giuseppe overlooks this odd behavior due to Katherine's overwhelmingly sweet, docile personality. She tells Stefan that she does not drink human blood because she does not desire power; for the same reason despite Klaus' offers, she has not exchanged blood with him since her initial transformation. She believes that drinking human blood should not be done lightly, and she wants to wait until she has found an eternal companion. Katherine sees a possible eternal love companion in Stefan and aspires to marry him. However, Damon's return from university halts any talk of marriage between Stefan and Katherine, as Katherine is also drawn to Damon's darkly, seductive charm. When she intervenes in a heated, intense argument between Damon and his father, Giuseppe, which would probably otherwise result in Damon being cut off from his inheritance, Stefan becomes slightly jealous and the pre-existing tension between the two brothers mounts. As summer begins to draw to a close, Damon notes that the Baron will soon leave Florence with Katherine, unless she chooses to marry and remain there. Damon also reveals that Katherine shared her secret with him, intensifying Stefan's resentment against Damon. The brothers begin to fight for her affections in earnest, but Katherine reminds them that she is a vampire and cannot live like a normal Florentine lady, and declares that her companion/husband must renounce ordinary human life for the shadows, meaning to become an immortal like her. Since this does not deter either of the Salvatore brothers, she tells them that she needs some more time to choose between them. Katherine makes her choice by coming to them separately during the night and exchanging blood with both, first with Stefan during lovemaking and then with Damon afterwards, turning them into vampires, declaring that she wants the three of them to be 'joyous companions, forever'.
Tragic Fate
Both brothers vehemently reject her decision, furious with jealousy, competitiveness and wounded pride. Katherine runs away from them in tears, distraught and deeply upset. She conceives a plan with her maid, Gudren's help that she believes will make Damon and Stefan put aside their intense hatred for one another and realize their error. As a result, she fakes her own death. The next morning, the brothers find what they believe to be Katherine's ashes and clothes, including her lapis lazuli ring which was used to protect her from the sunlight, beneath her favorite lemon tree. Stefan and Damon blame each other for her death, fetch their swords and kill each other by stabbing each other in the heart, waking up later in their family crypt as vampires.
Innocence Destroyed
Meanwhile, many years later, Katherine goes back to the German village to find Klaus, with whom she lives for centuries. The books suggest that he abuses her while teaching her to become a significantly tougher, stronger vampire, and over time with Klaus influence, she becomes power-hungry and insane, losing utterly her childlike innocence and naiveté. She eventually attempts to kill Klaus, evidently believing that she has succeeded.
Love Turned Into Insane Obsession
At some point during the late 20th century, Katherine arrives in the small American town of Fell's Church, Virginia, where she first sees Elena Gilbert, who bears a striking physical resemblance to her. Elena strongly resembles Katherine so much that Katherine even considers that she may even be a very distant relative of hers. Katherine subliminally influences both Stefan and Damon to follow her to the town, hoping that they will both see Elena, honor her memory and feel extreme guilt and remorse for the tragic events of the many centuries ago. However, Stefan falls deeply in love with Elena, who coincidentally turns out to be Stefan's soul mate, and Damon is also deeply drawn to Elena and tries to seduce her in order to win her from Stefan out of jealousy, revenge and hatred against him. Katherine masquerades as a white kitten which Elena's little sister Margaret takes as a pet. At times, Katherine acts antagonistically towards the brothers, as when she traps Stefan in a well and has serious thoughts about wringing Damon's neck while he is in crow form, a dream which Bonnie telepathically picks up on. When Stefan proposes to Elena and gives her the small gold lapis lazuli ring as an engagement ring that belonged to Katherine, the latter becomes extremely jealous and vengeful that Stefan has fallen deeply for Elena. As a result, Katherine drives Elena's car into the river, killing her due by drowning, not knowing that Elena had Stefan's vampire blood in her system.
Sacrifice
After Elena awakens as a vampire, Katherine disturbs Elena's memorial service by influencing the town's dogs to attack their owners outside the church. This event causes Elena, Stefan and Damon to realize that something is amiss; they begin searching for the 'Other Power' in Fell's Church, which leads them to a deadly confrontation with Katherine in the tomb of Honoria Fell where she has taken up residence, and attacks them in the form of a snow-white tiger. Katherine ties them up, tortures them a little and reveals herself to the trio, taunting them with the details of her antics in Fell's Church, of which they had been unaware. Meanwhile, she directs the dogs to attack students leaving the Snow Dance at Elena's high school. As she plots to kill them all, Elena outwits her by lunging at her and removing her lapis lazuli necklace, causing her to burn up in the sunlight, turning the centuries-old lie into the truth and saving the townspeople as well as both Stefan and Damon. However, Elena as a vampire, also dies alongside Katherine after being exposed to the sunlight. In Dark Reunion, Klaus arrives in Fell's Church seeking revenge against Stefan for Katherine's death.
Death
In death, Katherine did not simply disappear as the Celestial Court believed that vampires did. Instead, her continued existence after death gave her the time to reflect on her actions in life and led her to forgive Elena. Through unknown means, she also learned the truth behind Elizabeth Morrow (who is her birth mother), her relationship with Klaus, and the fact that she and Elena were part-Guardian and half-sisters. She regained the sanity that Klaus had beaten out of her during their century of companionship.
True Resurrection
In September 2014, Klaus used the blood of a number of vampires that he had changed to cast a resurrection spell, bringing back Katherine von Swartzschild to life. Shortly after her resurrection, Katherine fought against Stefan in a battle between Klaus' vampires (Cristian Sulez and ancient servants) and the supernaturally-aware students (Ethan Crane) of Dalcrest College - Stefan Salvatore, Elena Gilbert, Meredith Sulez, Bonnie McCullough, Alaric K. Saltzman and the Original Pack. While she believed that she owed Klaus, she did not have any intention of devoting herself to his cause any more than was necessary to repay her debt.
Katherine visited Stefan at his dorm, and coyly flirted with him. She told him that she was no longer angry at Elena, but Stefan did not believe her and threatened to kill her if she harmed any person at Dalcrest. She later talked to Damon at a football game, where she explained how she owed Klaus, before influencing the cheerleaders to dance a bassadanza as she and Damon had the day they met. They then went out hunting for humans to feed on. At the end of the series, Elena and Katherine appear to become more tolerant and understanding towards each other.
