First of all don't be shallow and concentrate only on her looks or only on her personality. Combine the two (65% personality and 35% looks). Second, of course you are going to have feelings for her. She is your best friend and I'm guessing you spent too much time together. It is bound to happen which is why there is a valid argument that states men and women can't be just friends. Finally, and most importantly, don't get involved with anyone unless you are sure it leads to marriage. Don't waste your time getting emotionally linked with a girl that is not going to be there forever. Your expectations for the next girlfriend will be higher, and higher for the next one untill your requirement for the woman that you would want to start a family with will be so high that no girl can meet that. So to answer your question in a short manner: don't act on this particular feeling. Not every feeling requires action. Hope this answered your question.