The small plastic body part on the ‘12-‘15 NC700X, roughly 6”x6”, behind the passenger seat is the one part on the bike that I find extremely difficult to remove by itself. The part locks into the two rear side cowls with tabs and plastic hooks. I firmly believe Honda messed up on the design of these interlocking parts. Often, removing that one rear piece results in breaking some of the tabs off.
Simply put, it is supposed slide rearward and then upward to separate it from the side cowls. If you put all your might into it, it still may not release.
Another way to get it off is to remove that tail cover and the two rear side cowls together as one assembly. To do that takes a little more time but is almost guaranteed to work. However, you need to remove the passenger grab rails first in order to do it.