Brenden really needs to start playing his own game, as it is obvious to everyone (except him) that Rachel is definitely playing for herself. If Hayden stays, he will go after Brenden. If Kristin stays, she will go after Rachel. Hence, Rachel REALLY wants Kristin out. Mr. PhD candidate better start thinking with the RIGHT head or he will be gone in a flash.
That being said, I really think Brenden is stuck at this point - he is obviously over the "Rachel is perfect in every way" phase, as he is constantly pointing out her shortcomings - her job, her tons of makeup, her drinking, her attitude and behavior with the other houseguests..... even if this "relationship" lasts through the rest of their time in the house, it definitely won't last outside the house.