To select a character other than Rex, use this chart to determine the key sequence to use during the title screen. When it shows two codes combined, press them at the same time.

Character - Key Sequence
Bonapart - Up, A, C
Edwina - Left, B & C, C
Robochic - Right, Up, Down
Stump - A & C, Right, C
Ramses - Right, Left, A
Webra - A & C, Up, Right
Guano - Up, Right, A & B
McFire - Down, C, Right
Skinny - Right, Down, Right
Sheba - B & C, B, B
Brainiac - Right, Right, C
Buff - Left, C, B
El Toro - B & C, A, Up
Spidra - A, Down, B
Weezil - Down, Right, Up
Mondu - A & B, Down, Left