Ardy Lightfoot

Cheat mode:
Hold Right + Y + L + A and press Start at the title screen. Press L during game play for invincibility. Press R during game play to jump higher.

Black and white Ardy:
Hold L + A while losing a life.

Old style Ardy:
Hold L + B while losing a life.

Small Ardy:
Highlight "Game Start" at the main menu, then hold Up + X + Start, in order.

Skip introduction level:
Press Start + Right + Y at the title screen.