Question

When you beat N in his Castle in BW 2, he says that he's going to find the trainer who beat him 2 years ago.After that scene, does he shows up again ?

Answer 1

Yes.You can see him again.Long after this, after visiting him at the Ferris Wheel in Nimbasa on Fridays, you will be able to rechallenge N as he wonders about the previous game's Trainer and if he achieved his dream. The team N uses changes on subsequent rebattles and all focuses on a different type of weather and are based on the season. Spring he uses his Water-type team, Summer his Fire-type team, Autumn his Sandstorm-team and Winter his Ice-type team. The battles are incredibly tough, and the weather is set up instantly using the lead Pokémon's ability. He can be rebattled once a season

Source

Answer 2

Yes.
He flees to another reigon in search of a strong trainer.
http://bulbapedia.bulbagarden.net/wiki/N
Pokémon depending on the season.

After catching or knocking out Zekrom B2 /Reshiram W2 , N arrives to:

congratulate the player and informs that Kyurem has returned to the
Giant Chasm. From then on, N will appearin Nimbasa City near the
Rondez-View Ferris Wheel on Fridays. After riding with him and having
caught or knocked out Kyurem, he returns to the ruins of his castle,
which now form part of VictoryRoad, for a rematch with different
Pokémon depending on the season. Spring Water. Summer Fire.

The battles are hard and he sets up his weather instantly using his lead Pokemon.

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Leads:

Autumn: Hippowdon
Spring: Poelitoad
Summer: Ninetales
Winter: Abomasnow

Answer 3

On fridays if you nock out N, Wait another season then go to the ferris wheel then once youve talked to N go to his castle To where you saw Reshiram and he is there with a different team depending on the season.