Win the World Championship and save the game. Then at the exhibition mode character selection screen, press Square, then press Square again and Jackel, Goober, Rascal, and Beastman will be selectable.

Main Event boxers in versus mode

Create a new boxer in "Main Event" mode, select, then immediately save a boxer to a memory card. Repeat this process to save a second Main Event boxer to the memory card. Then, begin a match in two player versus mode. Now at the character selection screen, press Square to choose one of the previously saved Main Event boxers.

Boxing trainer in versus mode

While you are at the two player versus mode character selection screen, press Square, then press Square again and the boxing trainer will be selectable.