Elena Gilbert bares as a striking resemblance to Katherine and that was the initial reason Damon and Stefan Salvatore were so drawn to her.
Damon revealed that she died when soldiers set fire on civilians in a church in 1864. Which was briefly mentioned during Elena and Stefan's History class. It was revealed that Katherine was a vampire during her stay with the Salvatores and was the one who changed Stefan and Damon.
It was thought that Katherine was alive and has been trapped with 26 other vampires in a mystical prison under a church for 145 years until Damon freed those trapped vampires. He learned to his great disappointment Katherine wasn't in the tomb. In 2010, it was revealed she had been free for 146 years. Anna, a vampire, told Damon that she last saw Katherine while running around the United States in 1983. She didn't care to look for Damon, even though she knew where he was at the time. So, it seemed that she was simply attracted to Damon, instead of in love with him.
Stefan stated that she was playful and knew how to have fun, but selfish and unworthy of love and forgiveness. She wanted herself, Damon, and Stefan to live with each other for a long time and she wanted to be with both of the Salvatore brothers, no matter what other people thought.
It was later mentioned that she was not dead, or in the tomb that they had been talking about, because she used a guard's obsession with her to convince him to free her in exchange for turning him into a vampire.
Later, it was revealed that Stefan and Damon were shot by Giuseppe, their father, while they were trying to save her. As Stefan lay dying Katherine lay next to him and professed her love for him.
It was later learned that Katherine wanted all of the vampires that were in the tomb dead. She sent Isobel to retrieve the invention which can be harmful against vampires. She also gave Isobel a lapis lazuli medallion, thus enabling her to walk in the sun.
At some point on Founder's Day, Katherine returned to Mystic Falls, pretending to be Elena. John Gilbert had set off the device and incapacitated the tomb vampires, including the Salvatore brothers. She posed as Elena and fooled many by her deception. Katherine pretended to be Elena and kissed Damon passionately. Jenna, Elena's aunt, interrupted them and unknowingly invited Katherine into the house. She then spoke to John, who told her why he despised vampires so much, believing her to be Elena. Katherine then took a knife and chopped off his fingers, along with his ring and stabbed him. Elena arrived not too long after and heard the noise. She went into the kitchen, curious as to what happened.
John Gilbert being seriously injured, Elena called for help and was terrified by Katherine's super-speed around the house before leaving. She wreaked havoc for Damon and Stefan, confusing them of who she really loved. Also, she threatened Bonnie, claiming that it never ends well for Bennett witches and Katherine choked Bonnie, but Bonnie used her powers on Katherine, like she did earlier on Damon. Katherine "acted" that she was in pain, but it didn't work because Katherine was stronger and immune to supernatural phenomena. She admitted to Stefan that she returned for him, but she was furious because he told her that he hated her. Enraged, she stabs him with a metal rod. Damon demanded the truth after he and Katherine kissed heavily, and she told him that she never loved him and that it was always Stefan that she loved. Later, she went to see Caroline Forbes and told her to give the Salvatores a message: "Game on." She killed Caroline by smothering her with a pillow, while knowing full well she still had Damon's blood in her system and would come back as a vampire.
Katherine appeared to Caroline as she was waking up, where she was mistaken to be Elena. She told her to not be frightened, and that they were going to have so much fun together.
Katherine revealed some shocking secrets to Stefan, as she made him dream that he was at the 1864 Founder's Ball with her. While there he noticed that Damon had found someone to entertain himself with. It was shown for that person damon had in his arms was Elena. Stefan saw, in his dream, that Damon and Elena were on a date and were kissing. Later on after the dream ended, Stefan thought he was holding Elena in his arms, but then realized it was Katherine. Later on, the two talked and Katherine proved that she never compelled Stefan to love her, his love for her being real, and she only compelled him to cover up his fear of vampires. Also, it was revealed that Katherine escaped the tomb with the help of a werewolf, George Lockwood, to whom after she got out, Katherine gave the Moonstone. In 1864, she also betrayed her best friend Pearl and all the other tomb vampires just to fake her own death. However, she was in love with Stefan, and genuinely cared about him. After the interrogation, she finally met her doppelganger, Elena.
It was soon revealed that she was seemingly in a relationship with Mason Lockwood, who she had been using to find the Moonstone for a currently unknown purpose. After Mason was killed, Katherine had compelled Matt to pick a fight with Tyler, so that Tyler would kill him and trigger the curse, because Katherine needed a new werewolf to replace Mason.
Katherine decided to retrieve the moonstone herself with the help of Lucy, a witch. She danced with Stefan but when he refused to hand over the stone, she killed Aimee Bradley. Caroline successfully lured Katherine into a room upstairs that Bonnie had put a spell on so vampires couldn't get out. In the attempt to kill her, Damon and Stefan partook in an intense fight between the three of them.
Lucy, however, had cast a spell over Katherine and Elena so whatever happened to Katherine would happen to Elena. However, Lucy turned the tables on her and worked with Bonnie, using a spell that incapacitated her. When she woke up, she was imprisoned in the tomb where Damon said she should have been for a long time. As Damon closed the door of the now sealed tomb, Katherine screamed that Elena was in danger and needed to be protected.
It was later revealed that Katherine did have a child before she was turned. Elena visited her and they had a conversation about her past, as Elena constantly gave her blood from a bottle to give her enough strength to talk. She told Elena that she was given blood by Rose to heal so she wouldn't die, and she later hung herself to be free. When Stefan finally found Elena, Katherine informed them that Klaus would kill all the people Elena loved, just like Klaus did to her. Later, Katherine was reading a Petrova history book with a drawing of her family with tears in her eyes, showing she had not completely lost her humanity.