Konami Code Hijinks
Go to any empty space and press the Konami Code: UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT, B, A, and you'll hear a jokey sound.

Unlock Alpha Warrior
On DREAM WORLD liberate MEMORY BRIDGE in the Yugi story

Unlock Black Luster Soldier
When you get the Black Luster Ritual use it on Gaia instead on the Dark Magician to get the Black Luster Soldier

Unlock Eye of Ra
To unlock the Eye of Ra, beat Marik while playing as Joey.

Unlock Joey
Win the story mode with both Yugi and Kaiba to unlock Joey

Unlock Silfer's Orb
To unlock Silfer's Orb, beat Kaiba's game.

Unlock Soul of Oblisk
To unlock the Soul of Oblisk, beat Yugi's story.

Get Obelisk The Tormenter
To get Obelisk the Tormenter, beat story mode once with Yugi Moto. He should appear as an item that you can summon once a round

Get the Magician of Black Chaos
To get the Magician of Black Chaos, use the black luster ritual on the dark magician