Damon and Caroline

Caroline and Damon's relationship began in the Pilot episode and continued romantically until Damon was vervained by Stefan and was locked up in the Salvatore mansion's basement. During the course of their relationship, Damon repeatedly fed off of her and compelled her. He did this to both, gain control of her and so that she wouldn't tell of what she saw. He flaunted their relationship and Caroline proudly exclaimed to her friends that she was dating the other Salvatore brother.

Season 1 

Damon and Caroline first meet at the Grill where, Caroline, feeling let down after Stefan rejected her for Elena, sees Damon and agrees to go home with him. The next moring, she wakes to find bite marks on her neck. She attempts to run but Damon hears her and blocks her entrance. She throws a lamp at him but he quickly subdues her. She asks him if he is going to kill her, he tells her he would eventually.

Over the course of the next few episodes, he continues to compel, drink from her and generally order and boss her around. He is shown to read one of her Twilight Saga books. He tries to get her to steal back a crystal that belonged to her friend, Bonnie Bennett. Damon knew that Bonnie could use the crystal to open a tomb that contained his, at the time, true love, Katherine Pierce and other tomb vampires.