Katherine Pierce
Katherine Pierce, portrayed by Nina Dobrev since season one, is one of the main antagonists of the series. Originally known as Katerina Petrova, she was born in Bulgaria and later banished to England after having an illegitimate child. Upon meeting Klaus and Elijah, two of the original vampires, she found out that she is a Petrova Doppelganger, which means that she has to be sacrificed to release Klaus' werewolf side. She used a vampire named Trevor, who'd fallen in love with her, to escape and tricked vampire Rose into turning her,making her useless to break the curse.Katherine's escape infuriated Klaus who vowed to find her and kill her for her disobedience. She then spent 500 years running and hiding to escape his wrath. Hundreds of years later, Katherine arrived in Mystic Falls, where both Stefan Salvatore and Damon Salvatore fell in love with her. When all the vampires in Mystic Falls were discovered and brought together to be burned to death, she escaped. During her stay in Mystic Falls she had given Damon and Stefan her blood which turned them into vampires after they were shot. Damon, believing Katherine was locked up in a tomb, spent 145 years loving her and trying to find a way to free her. When he is finally able to open it, he finds out that she is not in the tomb and was alive the entire time. In the last episode of season one, Katherine returns to Mystic Falls. In the first episode of Season 2,she tells Damon that she has always been in love with Stefan and tries to break up the relationship between Stefan and Elena Gilbert, Katherine's descendant and the current Petrova Doppelganger. It is later revealed that her real reason for returning is to get the moonstone and offer it and Elena to Klaus, in order to be free from him. upon learning of her plan, she is locked up in the same tomb by Stefan and Damon, which has a spell that keeps all vampires locked in it. When the spell is finally broken later on, she is compelled by Elijah to stay in. Elijah dies and then Katherine is freed, but chooses to start helping Stefan and Damon. When Klaus comes to Mystic Falls, he kidnaps her and keeps her in Alaric's apartment, where he tortures her. He eventually releases her, but she keeps track of Klaus and Stefan's settings. When she finds out that a vampire hunter named Mikael is the key to killing Klaus, she finds him and wakes him up, but when Mikael's plan to kill Klaus fails, she goes back into hiding. She has not been seen since her departure from Mystic Falls.