In Pokemon X and Y's Route 15, there is a building called the Lost Hotel, near the Dendemille Town Gate. Before getting to Dendemille, I entered and fought the gang members, but at the end, one gang member did not allow me to enter the room of the gang leader, and I thought that the room can be accessed post-game. After I defeated the champion, I went back and I still wasn't allowed in. What do I need to do in order to enter the room?
EDIT: So this is another question. According to one of the answers, you just get a trick. So what is the benefit of that trick?