Dean and Ruby

Dean Winchester and the demon Ruby have never been friends, nor have liked each other. But they have saved each other's lives once and awhile. Despite Dean's dislike for Ruby, he trusted her to a point, but not completely. When she come between him and Sam, due to Sam's demon blood drinking problem, Dean considers her enemy number one. When her betrayal comes to light, Dean is the one who kills her.

Background
Although Sam is the first to meet Ruby, he tells Dean about her, after she repairs the Colt for them. Following his hunter instincts about demons, Dean is furious that Sam let the demon live, and trust her. Dean believes deep down that all demons are bad ones. The two finally meet in Malleus Maleficarum when Ruby stops the Impala to give them a warning.