Although no cheat codes for "Yugioh Ultimate Masters" exist, the game features many different secrets to unlock including new themes, new battles, new cards and new challenges. Most of these unlockable portions of the game hinge on players having certain cards in their deck or completing certain objectives within the game.
Defeating all of the monsters in different tiers multiple times unlocks a number of different cards, many of them powerful. For example, defeating all Tier 3 monsters once unlocks Gilford the Legend. Defeating all Tier 3 monsters three times unlocks the Dark Eradicator Warlock and the Guardian Exode. Defeating all Tier 4 monsters five times unlocks Raviel, Lord of Phantasms and Horus the Black Flame Dragon Level 8, two of the game's most powerful cards.
Having certain cards in a deck can unlock many unique Theme Duels. For example, having Dark Sage, Dark Magician and Time Wizard cards in a deck will unlock the Dark Sage Theme Duel. Having Valkyrion the Magna Warrior, Alpha the Magna Warrior, Beta the Magnet Warrior and Gamma the Magnet Warrior in a deck will unlock the Magnetic Power Theme Duel. Winning 20 games in Survival Duel mode also unlocks a number of battle effects that players can turn on or off, including the elimination of flip effects and the constraint of only being able to use spell casters.