A poser one who does not follow his/her own ideas of what he/she likes. Following no one set norm makes you real and following no one set norm makes you poser. Poser is governed not by the clothes, music, or makeup you wear, but rather by the state of your mind and the conformity to your own beliefs. In short, a person who attempts to blend into a specific social group to which they do not truly belong.
At least, that's the real-world definition. In My Immortal, the definition appears to be "any person trying to be goff without Enoby's approval". A poser can look and act like a stanist goff and know all the popular lyrics of MCR and GC, but a real goff convert needs to go through horrible abuse and trauma. Or know a non-single or change pink roses to black.