Instructions
Locate Mindy in the "Tag Force" game. You need to interact with Mindy every day during the game in order to build trust.
Give Mindy a sandwich and talk to her. The sandwich and regular communication with the character will build trust and you will see the trust meter reflect this. When talking to Mindy, choose topics from the top three offered; avoid rumors.
Duel Mindy when you encounter her in the "Tag Force" or "Tag Force 2" game. You need to defeat Mindy in order to select her as a "Tag Force" partner. After you defeat Mindy, there will be a sound that indicates increased trust.
Go to your Slifer Red dorm room and access the database to find the list of duelists. Here you will find Mindy's name, once you have defeated her, and next to her name will be eight hearts. These hearts fill up as your trust with Mindy builds. When you are close to filling up all eight hearts, there is a good chance Mindy will become your "Tag Force" partner. You will be able to duel with a partner on Sunday in game.