After the player has defeated the Elite Four, Trevor can be found in Geosenge Town. There, he will tell the player about various Pokémon that can use Mega Evolution.
And you can also find him in the Battle Maison as a parter for Multi Battles.
After the completion of the story line, Tierno can be found in Coumarine City, asking to see a Pokémon which contains a randomly selected dancing move each day.
Tierno, like Trevor, can also be a Multi Battle partner.
After the player becomes the Champion, she will trade the player the unevolved form of her starter Pokémon in exchange for any Pokémon owned by the player.
As for after the trade (correct me if I'm wrong) I do believe she stays in Vaniville Town.
And since you know Calem.. That's it for X and Y.
Brendan/May in ORAS reside in their homes after the completion of the game.
Hope I helped.