Tyler
Tyler Smallwood, Robert E. Lee's resident bully and the first of Stefan's persecutors that we formally meet. At first he doesn't seem to have any real dislike of Stefan, only picking on him because he's a new kid at school and being an ass is what Tyler does best (gods love him!). His hatred cements itself, however, when he is humiliated by Stefan in a fight after he is found drunkenly assaulting Elena. His pride hit, Tyler pulls out all the stops in his efforts to bring Stefan down. When Mr. Tanner's body is discovered, it is he who riles the crowd up against his enemy and creates the air of distrust that causes problems for Elena and Stefan until Elena's mortal death. But even before that, he enlists the help of Caroline Forbes, using her contempt for Elena to devise a plan to chase Stefan out of town for good. After his plan is foiled and Elena is assumed to be dead, he feels the brunt of Stefan's rage as he drained of most of his blood in Stefan's preparation for his battle against Damon.

Tyler's contempt for Stefan is still present in The Dark Reunion, but is far less focused; probably due to the fact that he is no longer working out of his own interests, but under the ranks of another. He willingly aids Klaus in bringing Stefan back to town and leaves him to the will of his new alliance.

Damon
As I said in the article on Stefan's relationships, the air between these two brothers has always been tense at best. During their mortal life, friction was caused by Damon's blaming him for the death of their mother and a difference of beliefs on values and obligations. Later, they pit against each other more fiercely through their rivalry for the affection of first Katherine, then Elena. But when outside powers rise in the horizon, the pair try to set aside their differences and fight together. Damon outdoes himself on this score, proving on two occasions (once in The Fury and then in The Dark Reunion) that he'd rather stand by his brother than save his own skin.

Understanding Damon's dual treatment of Stefan: the need to protect and the need to put down, has never been a mystery to me. I'm sure many of you who have a younger sibling close in age to them can relate as well. Even when you love them to pieces, it's damn important to you that they realize that you are the one on top. I remember watching home movies of my brother and I when we were really young and Paul was just learning to walk. I was there to knock him off his feet every time he got up, as if to remind him to not be proud of his accomplishment because I was better. Or how when we were older, I lorded my 1 inch of extra height of him like a trophy, and then when he hit his growth spurt and soared to his 6'3 stature (I was devastated by the way), I changed tactics to reminding him how much smarter I was than him. To this day, if my brother knows anything he knows that his older sister's intellect can smash his to bits. And I love my brother. I can't ever stay angry with him and I'd do anything for him, but if I could go back to being that small child I wouldn't think twice about shoving him to the ground as he took his first steps. It's just what it means to be the oldest. And I think that was a lot of what it was between Damon and Stefan.

Katherine
Katherine's contempt for Stefan evolved from her despairing over the fact that her plans for reconciling the brothers had failed. She brooded over it for centuries, her newfound love for the joy of the kill only adding to her scarlet colored rage at the nasty turn of events. She lured Stefan to Fell's Church knowing that Damon would follow, hoping that they would see Elena, think of her, and feel hopelessly guilty and miserable. Insult was added to injury when her plans failed again: not only had both brothers fallen in love with her, but Stefan had asked Elena to marry him, giving her Katherine's ring. In the crypt scene at the end of The Fury, Katherine tells Stefan of her outrage that Stefan would give Elena her ring and that she hated him more because she had always loved him best. At this point, even if she was willing to let Damon live, her jealously brought her to lust for Stefan's suffering and ultimate death.

Klaus
Klaus's relationship with Stefan always puzzled me. LJ Smith was never really clear in The Dark Reunion as to why Klaus wanted to kill Stefan. I suppose he was just the most convenient vehicle for the last novel so she could bring Elena back to life (*gag*) and appease the readers that were dissatisfied with the original ending. But that answer doesn't satisfy my curiosities about Klaus enough.

Klaus' connection to Stefan was Katherine, who had become his companion after she left Florence, so it could be reasonable to say that he came to avenge her death. Klaus tells Stefan at one point that Katherine often used to make "unflattering comparisons" between the two of them and that she never really got over him, a fact that obviously infuriated him because he goes on to say that he beat her in response. Could that reaction have been out of jealousy? What other reason would he come down to Virginia to kill the one he felt responsible (since Elena was already dead) for Katherine's death? Also, if Katherine believed to have killed him, why would he let her get away alive with thephysical damage (temporary damage, but still) she had to have done to him? It could have just amused him, but if that were all, I can't see him going far out of his way to make the ones who hurt her suffer. Maybe it was love.

On the other hand, nothing we know about Klaus gives any evidence that he could become attached to a lover. He made several remarks about keeping Katherine because she was pretty and fun, but degraded any possibility of loving her mind. His comments about women are more degrading than Damon's ever were (though this was to bait Stefan more than anything else) and he certainly didn't show the state of mind that could foster an actual relationship.... There has to be some sort of side story here. And I want to see it, goddamn it!