What do you call someone who is seemingly working with you but is actually working against you?
closed as general reference by coleopterist, J.R., Andrew Leach♦, tchrist♦, MetaEd♦ Mar 14 '13 at 15:44
This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information. If this question can be reworded to fit the rules in the help center, please edit the question.
Depending on the situation backstabber may be an appropriate word. For example, someone may ask the advice of a friend regarding a love interest whilst that same friend has ideas of his/her own regarding the same person. The friend may seem to give good advice but is in fact trying to make the situation better for him/herself.
I would call that person dead weight.