I am trying to unlock the side quest but nothing. I've beaten the Elite Four and the Champion already.
After becoming Pokemon League Champion, beating Calem or Serena in Kiloude City, and powering up your Mega Ring in Anistar City, you can start the final quest at the Looker Bureau. Upon entering Lumiose City, you will receive a message on your Holo Caster from an incognito person, asking you to meet him in Rouge Plaza.
Do these things and it should work. Source: https://www.ign.com/wikis/pokemon-x-y/Looker_Bureau
