Katherine, played by Nina Dobrev, is the vampire that Damon and Stefan Salvatore both fell in love with when they were still human in 1864. Elena Gilbert is Katherine's spitting image, something that draws both of the brothers to her when they return to Mystic Falls.
Katherine was a playful and flirtatious young woman, but was also cold, calculating and possessed a wicked streak within her. Arriving in Mystic Falls in 1864 to reside with the Salvatores, she initially hide being vampire from both Stefan and Damon, posing as human orphaned during the Atlanta Campaign. She took delight in compelling their minds and playing them against each other to see who could win her over romantically. She said that her long-term plan was for the three of them living together, though present-day evidence suggests this failed dramatically. Damon tells Elena that Katherine died in a tragic fire,and also alludes to the possibility of her being one of the 27 local civilians trapped in old Fell's Church and burned alive during the Battle of Willow Creek. She was trapped in the burning church, but the 27 civilians were actually vampires.
145 years later, Damon reveals to Stefan that Katherine is not actually dead. All 27 vampires were entombed in a mummified state below the ruins of old Fell's Church.
Katherine had a handmaiden named Emily, a powerful witch as well as Bonnie Bennett's 4th-great grandmother through Grams. Pearl revealed Katherine was never in the tomb. A guard, obsessed with her, agreed to let her escape in exchange for turning him into a vampire. Anna tells Damon she last saw Katherine in Chicago in 1983 and she knew where Damon was and did not care.
Towards the end of the Season 1 finale, 'Founder's Day', Damon talks to 'Elena' as he's leaving the Gilbert house and he kisses her but they are stopped by Jenna, who tells her niece to get inside. Once in the house, 'Elena' encounters Jonathan Gilbert and they converse about the device. Suddenly, 'Elena' sees his ring and grabs a kitchen knife, cutting off his fingers and he realises that 'Elena' is, in fact, Katherine.