Katherine Pierce

Katherine Pierce, born Katerina Petrova, is a non-Original vampire and a main character of the series. She was also the second-known Petrova Doppelgänger of Amara. She was the main antagonist of Season Two, and later redeemed herself thanks in large part to her obsessive love for Hulk after countless failures to seduce Stefan Salvatore.

Backstory
In 1490, she was disowned by her entire family (except for her mother) because she brought shame upon her family after hooking up with her family's stable boy and giving birth to a baby girl out of wedlock when she was seventeen. Instead of keeping the baby, her family forcibly took the baby away before Katherine could see her newborn daughter. Her daughter was adopted by a noble family, and Katherine was then exiled to England as a punishment. She quickly assimilated into English culture and met two nobleman brothers, Niklaus and Elijah Mikaelson. At first she was attracted to both of the handsome men, but in time, she eventually learned that she was a Petrova doppelgänger and that Klaus was planning on using her as a sacrifice in order to break a curse that bound his werewolf side. She sabotaged his plan by running away with a moonstone, which bound his curse, and then tricked a woman named Rose into feeding her her blood so she could turn herself into a vampire.

Because Katherine was no longer human, her blood no longer was a viable component of the ritual to undo the curse. Klaus was so infuriated with Katherine's sabotage, that she was forced to live on the run for over 500 years as he hunted her down.

At some point between the 15th and 19th centuries, she took on the alias "Katherine Pierce" (which was an anglicized version of her birth name) and eventually met and befriended vampires Pearl, Anna, and a very powerful witch named Emily Bennett. In the late 19th century, she traveled to Mystic Falls, where she met Stefan and Damon Salvatore, whose family hosted her at the Salvatore Estate and let her live with them after she gave them a false story about being the sole survivor of a cottage fire that killed her entire family. She later fell madly in love with both brothers, and subsequently fed them her blood, resulting in their eventual transitions into vampires after they were killed by their father in the early autumn of 1864. When the Founder's Council found out the identities of certain vampires in town, she faked her death in a burning church and escaped being sealed in the tomb with the aid of George Lockwood, who helped her in exchange for the moonstone.

During the twentieth century, she continued to keep tabs on Stefan while still hiding from Klaus. In 2010, she came back to Mystic Falls with the intention of finally earning her freedom from Klaus by handing over the Petrova doppelgänger who came after her, a naive but beautiful girl named Elena Gilbert. To sweeten the deal with Klaus, she killed Caroline Forbes, who had Damon's blood in her system and later transitioned into a vampire, and tricked Mason and later Tyler Lockwood into triggering their werewolf curses, just so they could each also serve as the vampire and werewolf sacrifices necessary in the ritual. She even tracked down the moonstone so that she could give Klaus every component necessary to break his curse. However, when her plans ultimately fell through, she fled Mystic Falls again shortly after Klaus broke his hybrid curse on his own, knowing that he was not done punishing her for her betrayal half a millennium ago.

After failing to gain Klaus' forgiveness by securing the cure and handing it over to Elijah in Season Four, Katherine managed to draw Klaus away from Mystic Falls by revealing to him in a letter that a witch in New Orleans named Jane-Anne Deveraux was plotting against him. Once Klaus and Elijah officially returned to New Orleans, Katherine moved back to Mystic Falls, confident that the Mikaelsons would be too preoccupied with their complications there to pay any attention to her.

Katherine would eventually meet Hulk and Carol Danvers, falling in love with the former so much that she began to redeem herself and became Hulk's lover.

Katherine's notable ancestors include Amara, who was Silas' true love and presumably the world's first immortal woman, and Tatia, whose blood was involved in the creation of the adapted spell of immortalitythat was cast on the Mikaelson Family by Esther, as well the curse that bound Klaus' werewolf side. She is also the maternal ancestor of Isobel Flemming and that of Isobel's biological daughter, Elena Gilbert. She was once the romantic interest of Stefan and Damon Salvatore, as well as Mason Lockwood. Since the late 15th century, she has also been the romantic interest of Elijah, with whom she was involved in the 21st century until he effectively ended their relationship by leaving to join his half-brother, Klaus, in New Orleans.

Trivia

 * Katherine might be bisexual, as she had sexual relationships with many women before even meeting the Salvatore's. Later in the series, she's revealed to be bisexual after Hulk reads her journal and learns she had an affair with Queen Elizabeth.