Pokémon Trainer Cynthia

Like previous games, Pokémon Black & White include some ties to the past games. This time, Sinnoh Champion Cynthia makes a return appearance. In this game, you will find her in a house in Undella Town. There, she will tell you about Sinnoh and then will challenge you to a battle. This is a full six on six battle and she has many powerful Pokémon

 
Pokémon Trainer Cynthia Spiritomb Milotic Lucario Braviary Shiburodon Garchomp
  Level 75 Level 75 Level 75 Level 75 Level 75 Level 77

After you have defeated her, you'll have the ability to come and rematch her again at any point. However, in addition to this, various other characters come to her house including Elite Four members Shauntal and Caitlin, the latter of which reminisces about the Battle Castle in the Battle Frontier of Sinnoh.