Alder - Unova's Champion and the strongest trainer. He will become available for battle after you beat the game for the first time. Without N and Team Plasma interfering, now this is your chance to face him. But beware, the Elite Four has also gotten much stronger and they added 2 more Pokemon in their party

Cynthia - Yes, you can face Cynthia, Sinnoh's Champion. What's she doing here? Investigating. She will become available for battle after you beat the game for the first time. She will be at the house at Undella Town. Remember: she still has her all-mighty Garchomp. You can face her anytime once per day at Spring and Summer only. Sometimes, two of the Elite Four, Shuntal and Caitlin will come to the house where Cynthia is at in Summer.

Game Freak Morimoto - He is the designer of the game and he is a real person who made this game (both Black & White). You will take a notice that his sprite is actually a Veteran Trainer. Be prepared: because you are about to face him with a very high leveled Pokemon. He isn't too hard but he can be tricky with his party. He will become available after you beat the game for the first time. He can be found at Castelia City on 22nd floor. You can face him anytime once per day everyday.

Note: The name "Alder" is only part of the name of the user here, Aldermar. Ironic isn't it?