After defeating N and becoming the champion, you won't find him directly in the game. He leaves Unova after his defeat and his whereabouts are not revealed.
However, there are some theories and interpretations surrounding N's fate:
* He might have returned to the Pokémon he released: N's main goal was to free Pokémon from human control, and his actions in the game suggest he might have returned to live among them.
* He might have gone to a different region: N's connection to Team Plasma implies he could have traveled to another region to continue his ideals.
* He might be waiting for the player in a future game: Some speculate that N could reappear in a future Pokémon game, perhaps as a rival or ally.
Ultimately, the exact fate of N after the events of Black and White is left open to interpretation. It's up to the player to decide what happens to him after the game's conclusion.