You can't well at least until they come up with the 3rd game.
You can beat cynthia beat the Pokemon league again find the legendaries complete the poked ex and your basically done with the game
you can find it in black city after you find the seven sages and beat the elite four and the champion
he only is in Pokemon platinum and Pokemon black and white in Pokemon platinum you see him in jubilife city, and I think maybe at Team Galactic HQ. in Pokemon black and white after you beat the elite four, he is looking for the seven sages. after you find all seven sages he'll talk to you and he'll arrest the 7 sages. you will never see him again after that.
Complete the Pokedex and try to get a black trainer card.
looker only comes when you beat the game and he gives you a Super Rod and you meet him when you find the seven sages.
To get the super rod you have to beat N and Looker will give it to you when he asks you to find the 7 sages.
im pretty sure you cant find him after you beat him at n's castle
you have to beat the galactic warehouse, and when/if you beat it you then go into the galactic HQ by entering from the galactic warehouse, and then Looker will come open the door.
Only if you beat the Pokemon league.
u can beat it! u have to kill the elite 4 then u find all the sages, then u fill ur pokedex and fight every trainer.
beat Cynthia at undella town complete unova and national pokedex get dive and go into the abyssal ruins at undella bay find the seven sages and find and beat cheren and bianca one more time.
The Elite Four in Pokemon Black/White is quite simple to beat (in comparison to the other games), but what you have to do next is defeat N, the the leader of the Seven Sages, whatever his name was.