Katherine Pierce was first introduced in The Vampire Diaries through flashbacks, and viewers were forced to formulate their opinion of her by way of Stefan and Damon's memories. After showing up during the present day and becoming a reoccurring part of the cast, Katherine painted a better picture of who she was through her words and actions.
She was immediately portrayed as a vain character who only cared about herself, but it became clear through the series that she had several layers. Katherine was more than capable of love, and she was forced to become cold and manipulative to survive. She managed to hold onto some semblance of her humanity, which is proved by her romantic gestures in the series.
Watching Over Stefan
For a century and a half, Stefan was under the impression that Katherine had died during the fire in Fell's Church. But as Damon learned relatively early on in the TV series, Katherine was still alive. Because she was on the run from Klaus, Katherine had to be careful where she went and how long she stayed.
Despite the threat it posed, Katherine checked up on Stefan throughout the years to ensure his well-being. Stefan didn't learn of her actions until the 21st century, but it was proof that Katherine was willing to put herself at risk to see the man she loved, even if it was from a distance.
Bringing Damon The Antidote
There are reasons why Elena should have been with both Salvatores in The Vampire Diaries, as there was a period when she had feelings for Damon and Stefan. Katherine said multiple times throughout the second season that she had only ever loved Stefan, but she ends up contradicting herself.
Katherine was finally free of Klaus's compulsion, and if she wanted to survive, the wisest thing to do would be to get out of Mystical Falls immediately. Stefan thought Katherine put her own self-preservation before Damon, but she proved him wrong in a surprising turn of events. She gave up precious time in order to save Damon's life and admitted that she did love both Salvatores.
Locking Herself In A Coffin With Stefan
It could be argued that Katherine thought she had nothing to lose, but by locking herself in a coffin with Stefan, she was putting her life in danger. It would only take a second for Stefan to lose control and kill Katherine, as she stood no chance against him as a human.
But Katherine truly did love Stefan, and she was willing to take the risk to help him through his trauma. She continued to rile him up, no matter how many times Stefan warned her, and refused to leave the coffin until he overcame his claustrophobia. It wasn't the wisest plan, but it was successful.
Saving Stefan's Humanity
There are several scenes that prove Katherine and Stefan were soulmates in The Vampire Diaries, and this was a big one. Choosing Stefan's humanity over her own life was one of the most selfless things Katherine had ever done. After five hundred years of running, she finally had the chance to get rid of Klaus for good.
However, she realized that Klaus's death would result in Damon's, and while she did care about the eldest Salvatore, it wasn't the reason why she made the sacrifice. Katherine knew that if Stefan let Damon die, he would never get his humanity back. She was determined that the man she loved existed under the emotionless facade, and Katherine gave up the chance to finally live in order to bring him to the surface.
Giving Elijah The Cure
Katherine choosing to give the cure to Elijah was a plot twist that no one saw coming in The Vampire Diaries. The cure was the only thing Katherine had to bargain for her freedom, so it seemed unlikely that she would ever hand it over willingly. Katherine is a person who sold out her friends and found family without blinking in order to ensure her own survival.
However, she was so desperate to prove her love to Elijah that she gave him the only thing that might make him understand how she felt about him. Elijah granted Katherine the second chance no one else would, and by handing over the cure, she proved that she cared about his opinion of her more than her own life.
Saying Goodbye To Stefan
Katherine's goodbye to Stefan could be considered her most romantic scene. It was especially impactful due to Stefan's voiceover, in which he proclaimed that Katherine was incapable of love. Stefan is unknowingly proved wrong while he's speaking, as a flashback shows Katherine running back towards Stefan's body to say goodbye.
Every moment was vital to get her out of town alive, but she stopped to tell a lifeless Stefan that she loved him. Although he couldn't hear her, she promised they would be together again and kissed him goodbye. It's especially heartbreaking because Stefan never found out about Katherine's last words to him.
Helping Elijah Believe In Love
Elijah is one of Katherine's best love interests in The Vampire Diaries, as he continued to see the good in her when no one else did. While Katherine was the more cynical of the two in the present, it's easy to forget that there was a time when the roles were reversed.
Elijah admitted that he didn't believe in love, and Katherine was able to change his mind by posing the question, "If we cease to believe in love, why would we want to live?" If Katherine hadn't been able to get through to him, it's likely that Elijah wouldn't be the caring and righteous man he became.
Going Back For Stefan
Katherine had several reasons for going to Mystic Falls, but Stefan has always been one of the most important. Stefan never believed that Katherine truly loved him, and he assumed that she was just using him as an excuse to keep her true plans under wraps.
Stefan wasn't completely wrong, as Katherine always had ulterior motives, but she wasn't lying when she said she came back for him. As proven throughout the series, Stefan is the love of Katherine's life and she always intended to seek him out again when the time was right.