Once you have Mixed Records with a friend, you get all the Secret Bases on their cart. Your cart can store up to 10 secret bases. In the secret bases you can battle your friends inside and get Experience Points. They have the same Pokémon as your friends did when they Mixed Records, however their Sprite is not the normal Sprite. Here is a list of the Trainers and how they are picked:

ID ends with...
0 Male Youngster
0 Female Lass
1 Male Bug Catcher
1 Female Student
2 Male Rich Boy
2 Female Lady
3 Male Camper
3 Female Picnicker
4 Male Cool Trainer
4 Female Cool Trainer
5 Male Youngster
5 Female Lass
6 Male Bug Catcher
6 Female Student
7 Male Rich Boy
7 Female Lady
8 Male Camper
8 Female Picnicker
9 Male Cool Trainer
9 Female Cool Trainer