Out of my own experience by using the DS download station I was able to find four secrets not found in the game:
1. Bro. 1, Name 1stMega Man: You can brother force with the original Mega Man!
2. Bro. 2, Name Legend Master Shin: You can brother force with Legendary Master Shin!
3. Card 1, Name Ra Mu: You can gain the power of the final boss, Ra Mu in a card form!
4. Card 2, Name King Grandeur: You gain the power of Tribe King in a card form!
Check your local Gamestop or your favorite game store for a DS download service system.