To unlock the four secret characters (I shall refer to them as Alex, Harry, Nina and Kate) you will need a N64 Transfer Pak and the GBC Mario Tennis. Once you have connected these things to the N64 controller, turn on the N64. Follow the prompts, and you will unlock Alex, Harry, Nina and Kate in 3D! You can earn points for them in their new home, and send the points back to the GBC version to level them up. They can be any of the types (all-around, power, speed, technique, etc.) depending on how you level them up. You may resend the GBC characters as you level them up to improve the N64 characters performance.