Here are some details concerning the Challenge and Assist Keys in Pokemon Black and White 2:

- Black 2 gets the Challenge Key
- White 2 gets the Assist Key
- unlock these keys at the end of each game
- share the key with another game/player
- keys make the game more challenging or easier

I would rather have the Challenge Key so I could make the game harder. Which Key would you rather have?

Source: gonintendo.com