When you have beat the game go to the Title screen and enter the following codes quickly before the Demo starts to unlock the corresponding characters.

Unlock Diablo Tiger, Cyclopes Type One/Two Zoid Data and Zoid Cores:
L, L, R, R, Up, Down, Up, Down, Left, Left, R, R, Right, Right, Left, Up, Start

Unlock King Gojulas, Gilvader Zoid Data and Zoid Cores:
R, R, L, L, Down, Up, Down, Up, Right, Right, L, L, Left, Left, Right, Down, Start