There are quite a few unlockable items in Yu-Gi-Oh! 5D’s World Championship: Over the Nexus for the Nintendo DS.
Here is the full list of unlockable items, and how to unlock them.
5D’s Standard Disk – Beat the game
Blackbird – Get an “S” rank in all Battle Royale matches
Different Facial Expressions – Win a duel against Torunka
Duel Phantom (Duel Runner Frame) – Talk to the broken Ghost with Misaki as your partner
DWE-CG – Talk to Klaus after beating the game
DWE-X – Talk to Jack and complete his event
Golden Shield Disk – Talk to Ransborg after beating, and finish his event
King Replica Model – Talk to Jack after beating the game
KPC-000 – On the fourth floor of the apartment (requires the Lucky Key)
Lambda Frame – Talk to Mina with Misaki as your partner, and complete her post-game event
Leather Top and Bottom Set (Yusei’s Outfit) – Located behind Team 5D’s garage
Leo’s Custom Disk – Search outside of Sector Security
Lucky Key – Talk to the guy behind Team 5D’s garage, and select the right box
Missing Ring – Duel all three Computer Ghosts
Speed Spell (Deceased Synchron) – Found as a Blue card on Battle Royal Race
Stuffy Collar Shirt – Duel all three Computer Ghosts
Suit Style – On the fourth floor of the apartment (requires the Lucky Key)
The Spirit World – Duel all three Computer Ghosts
Waiter Outfit – Talk to Stephanie and complete her event
Wheel Of Fortune – Get an “S” rank in all Battle Races
Yusei-Go – Get an “S” Rank in all Time Races