Enter 735409784657 as a password to get all God cards.

When going to places in the first city, like the shop, time can be saved by hitting the "Home" icon in the bottom-right corner. For some reason, this doesn't cause any time to pass, and allows you to get much more done in one day.

After winning the Expert Tournament and get the title of The King Of Gameslook for Ishizu at the West Town Zone once you find her, she will make you battle Kaiba in a single duel; if you beat him he will hand a God card to you. If you choose Wisdom he gives you Slifer The Sky Dragon and if you choose Power he gives you Obelisk the Tormentor.

You must progress through the game until you get the Marik. After you duel him a lot, and if you have either Slifer or Obelisk in your trunk, he will ask to trade you for his Ra.