Loneliness Stefan, that’s why you and I memorialize our dead. There’s the briefest of moments where we literally hold their life in our hands and then we rip it away and we’re left with nothing. So gathering other peoples letters or writing their names on a wall is a reminder that we are left infinitely and utterly alone.
Klaus and Stefan first met in Chicago in 1922 when Stefan was a Ripper and dated Klaus' sister Rebekah. In the beginning Klaus did not like Stefan, but this changed when Klaus discovered that Stefan was a Ripper and they became good friends. When Stefan, Klaus, and Rebekah were attacked at a bar by Mikael, with wooden bullets, Klaus knew that they were not safe anymore so he compelled Stefan to forget him and Rebekah. Klaus thanked him for showing him what it was like to have a brother once again, leaving Stefan without any memory of him or his sister.
Stefan and Klaus met again when he was in Alaric Saltzman's body at the Gilbert house. As Stefan stops him from killing Jenna, Klaus tells him that it would be easy for him to take his revenge if Elena tries to stop him from breaking the curse. Stefan screams at Jenna to leave and gets his vampire face on before he beat Klaus unconscious.
In The Sun Also Rises, Stefan tried to change places with Jenna who was turned into a vampire, but Klaus kills her to break his curse and injures Stefan. Klaus tells Elena and Stefan that he doesn't want Stefan dead because he has other plans for him.
In As I Lay Dying, Stefan is looking for Klaus to heal Damon's werewolf bite. Klaus makes a deal with him, showing interest in Stefan. He says that he had heard of Stefan or at least the Stefan who had been a Ripper whenever he lost control of his urges. Wanting the 'talent' that came with being a ripper that Stefan posed, Klaus promised to give him the cure for the werewolf bite; his blood, the blood of a hybrid; if Stefan did everything he asked. During the treatment, Klaus forces Stefan to drink human blood which caused him to lose control. Satisfied with Stefan's cooperation, Klaus had the cure delivered to Damon and told Stefan that they would be leaving town.
Before they left however, Klaus wanted to make sure that Stefan honored their deal and brought out a human girl whom he bit before letting her go. He also said that he hadn't compelled her to behave since a 'real ripper enjoys the hunt'. Stefan was unable to control himself and he killed the girl. A pleased Klaus then said that they could leave.
Stefan and Klaus have tracked a werewolf named Ray Sutton for two months. When they found Ray, Klaus made Stefan torture him to tell them were his pack was. When they find out that Damon is on his way to find him, Stefan goes to stop him. Stefan came back and Klaus got the information they needed, giving Ray his blood and killing him so that he could become a hybrid.
In The End of The Affair, Klaus tells Stefan about the 1920s and how he had a relationship with Rebekah, Klaus' sister. Stefan used to look up to Klaus as his brother and best friend. But when a police officer with wooden bullets came in,
Klaus and Rebekah had to leave because they knew who was coming after them, so Klaus compelled Stefan to forget them until he wanted him to remember. In 2010, Stefan finds a picture of Klaus and him from the 1920s and didn't remember being there with him. He only remembers him being the 'hybrid dick' who killed his girlfriend Elena. At the end of the episode Klaus made Stefan remember him and Rebekah.
In Disturbing Behavior, Klaus and Rebekah are trying to find where the necklace is. When Stefan talks to Rebekah about Mikael, Klaus comes in to say that Gloria has moved on, but to Klaus' surprise Rebekah and Stefan were having a deep discussion. Rebekah tells Klaus that Stefan is not with them so Klaus sees what he is hiding. When Stefan wakes up Klaus knows everything and he has brought Stefan back to Mystic Falls to make sure that Elena dies this time.
In The Reckoning, Klaus and Stefan are in Mystic Falls. Due to Stefan betraying Klaus, Klaus was breaking Stefan's neck all afternoon for his anger towards him. When Klaus gets a hold of Elena, he tells Stefan to kill the other people in the room, but Stefan doesn't want to so Klaus compelled him to attack them and he does. After he drinks their blood, Klaus gives Stefan a timer; if they don't find a way for Klaus to make hybrids in twenty minutes, Stefan will have to feed on Elena. When the timer hits 0:00, Stefan tries not to attack Elena and tells her to run. When Klaus catches Stefan he tells him to forgot his humanity. Stefan responds with no, pushing Klaus away. Klaus then angrily pushes Stefan against the wall, screaming at him to shut it off. When that didn't work, he compelled him to forget his humanity. Elena asked what Klaus had done, with him responding that Stefan is "fixed". Stefan attacks Elena, but Klaus stops him, realizing that the blood of the Petrova doppelgänger is the only way to make his hybrids. At the end of the episode, Stefan is still in Mystic Falls because Klaus gave him a mission to protect Elena. Stefan continues to protect Elena as he was told to do by Klaus.
InHomecoming, after Klaus kills Mikael, he gives Stefan back his freewill by removing the compulsion placed on him and Stefan responds by betraying Klaus and stealing the coffins containing the bodies of Klaus' family.
By Season 4, their friendship is still strained, but appears to be repairing as they decide to team up together during the episode The Five due to their mutual interest to finding the cure. They manage to find out where the sword is which would help them and then Klaus swears Stefan to secrecy as he got the sword they needed to unlock the cure. But this secret mission does not go well, when Stefan tells Damon about the cure, and by extension, many others also find out, to Klaus' dismay, meaning once again their friendship was tested and did not end at a good place. It seemed they still did not trust each other enough to be a team, mostly on Stefan's part. Stefan did not share Klaus' optimism for a good friendship between them. Klaus went some time believing Stefan was happy to work with him and there was no longer any animosty between them.
But Stefan did not trust Klaus with the sword and found that he broke in to his house to get the sword which he did not find. When he questioned Klaus, who had assumed to that point he had the complete trust of Stefan and their old friendship back, he found Klaus to be angry with what he did when he confessed so he declined to tell Klaus that his hybrids had turned against him as they were not sired anymore and had planned against him to ensure he is dead and buried. Stefan walked away from Klaus who had given Stefan the oppurtunity to tell him the truth about the hybrids betrayal, which Stefan did not, and Klaus knew Stefan had walked away from him expecting him to be dead by the end of the night.
As Klaus dealt with these betrayals, Stefan spoke with Caroline that night about how guilty he felt for turning away from Klaus and not even attempting to save his life, questioning why he thought he and everyone else was better than him when they were not. He finally acknowledged that this was because Klaus was alone, as Klaus had confessed to him that day, showing that Stefan no longer completely hate Klaus anymore, although it took Klaus some time to accept that he had betrayed him.
As time passes, although they still had not spoken the next episode, it is still not known whether or not Stefan still agrees with his conclusion, but he teams up with one of Klaus' siblings to find the cure instead, who he knows wants to use the cure to upset Klaus, meaning that perhaps he does not completely care for his former friend, an assumption until they speak again.
Klaus: The only thing stronger than your craving for blood is your love for this one girl.
Klaus: I don't want anything from you. (Walks To Elena's Bed) Oh! On the other hand, if you had told me a few hours ago, I wouldn't have cared a lick about her dying. But as it turns out, (Bites His Wrist) you may be of use to me after all.(Feeds Elena His Blood) Here we go.
Stefan: It's a good thing. I have nothing to do today, except get answers out of you.
Klaus: Fine, you might be actually useful in persuading my sister to cooperate. The brotherhood of the five was a group of highly skilled vampire hunters. We crossed paths with them in the 12th century,Italy. My siblings and I had followed the Normans as they conquered the south. Eating, turning people as we went. But with the blood shed came exposure.
Stefan: So, these hunters have been around for nine hundred years?
Klaus: Apparently, though our friend in the other room is the first I've seen since then. Kinda makes you wonder what they've been up to all these years.
Klaus: Get Rebekah over here. She's being stubborn and hateful, I..I need to make peace with her. I want her to give me some very important information about the hunter which she won't do unless she believes she's made up.
Klaus: She knows where it is. And you're going to get her to tell you. You have a chance to save Elena from the very thing that's going to destory her. You can call it a deal with the devil if you like, but you know you won't walk away from it.
Klaus: I have a plane waiting. I'll get the sword. Mu hybrids will keep an eye on Connor. You put Rebekah no one will find her and make sure Jeremy forgets everything he heard today.
Klaus: No one must know about the cure, do you understand? Not Damon, not Elena, nobody. If this gets out, it could mean all our deaths. So, if you tell anyone, I'll erase each and every memory from all of....