Access to the Lost Hotel in X and Y?

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?

Solution 1:

Once you've received the roller skates and unlocked the northern half of Lumiose City, you can learn the tricks from roller skaters in the area. Here are their locations:

Trick #1: Backflip - By a tree just outside of the Hotel Richissime on North Boulevard.
Trick #2: Parallel Swizzle - Inside an office building on the western end of South Boulevard.
Trick #3: Running Start - By the monument in Vert Plaza.
Trick #4: 360 - Inside Cafe Rouleu on Estival Avenue.

With all four tricks learned, return to the man in Lost Hotel. He will let you through to see the Boss, who teaches you the game's fifth and final roller skate trick - the Cosmic Flip.

Source here.

Solution 2:

You need to learn all of the rollerblade tricks from the npcs in Lumiose city, then the punks will let you through and you will learn the final rollerblade trick.