The story of Stefan's love and hatred for Katherine is what sets The Vampire Diaries into motion. It was difficult to get attached to the relationship during the first season, as Katherine was only shown through flashbacks and her relationship with Stefan appeared to be based on manipulation.
However, when Katherine showed up in Mystic Falls, viewers got to know her on a deeper level. Katherine was made out to be a villain from day one, but her love for Stefan was proof that there was still something human inside of her. No matter how many centuries went by, Katherine never stopped loving the younger Salvatore, and despite all the horrible things she had done, Stefan couldn't stop himself from caring about her.
Katherine Makes A Deal To Keep Stefan Safe
Katherine never does anything without an ulterior motive, making her an untrustworthy ally. When she stays at the Salvatore Boarding House and decides to aid Elena and the brothers, Damon grows suspicious of Katherine's helpfulness.
When Damon is almost killed he realizes that Katherine knows more than she's letting on, and she admits that she made a deal with John and Isobel. If Katherine stayed in town to help them defeat Klaus, they would spare the life of one of the Salvatore Brothers. Katherine stays to protect Stefan, despite being on the run from Klaus for over 500 years.
Stefan Convinces Katherine To Fight
After spending centuries on the run and determined to survive no matter the cost, Katherine found herself helpless to stop her impending death. Elena force-fed the cure to Katherine and turned her into a human, which caused her age to catch up with her. Katherine had been alive for over half a century, and she was quickly becoming old.
Katherine's been through hell and survived, but this was was an unbearable fate for her, but Stefan tells her, "You're Katherine Pierce. Suck it up." Afterward, Katherine fights to keep living.
Katherine Goes With Damon And Elena To Find Stefan
As a human, Katherine was weak, vulnerable, and left without protection from the countless number of people who wanted her dead. Still, when she and Elena both have the same dream about Stefan, she joins her and Damon on a road trip to track him down.
Katherine admits that she cares about Stefan and she always has, which isn't a lie. Throughout the show, Stefan was the only person Katherine wouldn't let anything happen to, even if it meant putting her own life on the line.
Katherine Helps Stefan With His Pain
After Stefan is locked in a coffin and thrown in the quarry by Silas, he spends three months drowning and coming back to life. He suffers from the memories afterward, and starts to panic anytime he's trapped in small spaces.
Caroline decides to face the situation head-on and forces him to face his trauma. Katherine puts her life at risk and locks herself underground with Stefan and is able to ground him. The two almost kiss before Caroline opens the door and ruins the moment.
Stefan Sees Who Katherine Truly Is
Stefan sat by Katherine's bedside as she lay dying and was the only person who offered her comfort in her time of need. Stefan was genuinely upset about what was happening to her, seemingly, the only one.
Damon took pleasure in tormenting Katherine and told her she was responsible for her family's slaughter back in 1490. Stefan took away Katherine's pain from that night and told her that none of it was her fault. She was just a 17-year-old girl who lost everyone she cared about.
They Give Into Their Feelings
When Katherine becomes human, there starts to become an obvious shift in the way she treats people. She's still out to survive, but she starts making more selfless decisions and even puts her life at risk for those she previously didn't care about.
Stefan notices the change in Katherine and it causes them to grow closer after his breakup with Elena. Katherine helps Stefan through a difficult time and they kiss which leads to the two of them having sex. Stefan admits that he isn't ready to forgive Katherine for the past 140 years, but when she walks out the next night, he stops her and holds her hand.
Katherine Saves Stefan From Gloria
Katherine seeks out Stefan when he's in Chicago with Klaus, hoping to get in on whatever plan he's forming to take Klaus down. Stefan refuses to accept her help and is kidnapped by Gloria, a witch who's friends with Klaus and wants to figure out what Stefan is hiding.
Gloria uses her magic to torture him until she discovers that Elena is still alive, which is presumably why Klaus can't make his hybrids. Before Gloria has a chance to turn Stefan over, Katherine kills her and saves his life.
Katherine Gets Through To Stefan When His Emotions Are Off
As Stefan brought up, Katherine wanted Klaus dead for the past 500 years so she could finally stop running. But when the opportunity arises to kill him, she passes it up to save Damon's life. Katherine is a character who's known for putting herself first, and Stefan questions why she would give up her freedom for Damon.
She admits that it wasn't just about saving Damon, it was about saving Stefan's humanity. Katherine went to Stefan when she found out Damon was in danger, and she was the only person who could break through to him when his switch was flipped.
Elena Suspects Stefan Still Has Feelings For Katherine
With Stefan and Damon's help, Elena finally turns her emotions back on after Jeremy's death. She immediately clings to her anger for Katherine and puts all her energy into finding a way to kill her. Even though Katherine was the one who killed Jeremy, Stefan goes out of his way to stop Elena from killing her.
Stefan claims that he was trying to save Elena from herself, which is true, but Elena was quick to accuse him of still having feelings for his ex. Stefan can never fully let go of Katherine no matter what she does, so there's likely some truth to what Elena said.
Katherine Checks Up On Stefan Through The Decades
Stefan had spent the decades after Katherine's alleged death believing their entire relationship was a lie. What he didn't know was that Katherine was still alive and that she consistently checked in on him to make sure he was okay.
Katherine never revealed herself and didn't interfere with his life until she came back to Mystic Falls over a century later. Most people believed that Katherine was incapable of love, but her devotion to Stefan was proof that they were wrong.