Sonic Rivals Cheats

Unlock Metal Sonic
To unlock Metal Sonic you must beat the game with Sonic, Knuckles, Silver, and Shadow. (In no particular order).

Unlock new cups
To unlock new cups (as well as obtain Chaos Emerald cards) for Cup Circuit mode, you have to complete Easy, Medium, and Hard challenges in specific zones in Challenge Mode as follows:

Blue Chaos Cup: Complete all challenges for Death Yard Zone
Cyan Chaos Cup: Complete all challenges for Colosseum Highway Zone
Green Chaos Cup: Complete all challenges for Forest Falls Zone
Grey Chaos Cup: Complete every challenge in the game
Purple Chaos Cup: Complete all challenges for Meteor Base Zone
Red Chaos Cup: Complete all challenges for Sky Park Zone
Yellow Chaos Cup: Complete all challenges for Crystal Mountain Zone

Unlock Skins
In order to unlock certain skins you must complete story mode with certain characters:

Knuckles Skin: Complete story mode with Knuckles.
Metal Sonic Skin: Unlock Metal Sonic.
Shadow Skin: Complete story mode with Shadow.
Silver Skin: Complete story mode with Silver.
Sonic Skin: Complete story mode with Sonic.