Instructions
Play as the 14 original characters available to you at the start of the game. Use the characters in any regular mode, such as Versus. When you use all 14 characters, Marth unlocks. Alternatively, unlock Marth after you play 400 Versus battles.
Go to the main menu and select "Classic Mode." Select Marth as your fighter. Complete Classic Mode without using any Continues. After the credits, Roy confronts you.
Battle Roy and defeat him to unlock him as a playable character. If you lose the fight, you must replay Classic Mode and try again.