Instructions
Collect 10,000 rings by playing stages in story mode. Any rings you collect are added to your bank when the stage is complete. You do not need to play as Shadow to gather rings; all character's rings are stored in the same bank.
Using Shadow, enter Chao Garden and walk across the hall to the Chao Kindergarten. Open the closet inside the school to visit the Black Market.
Exchange 10,000 rings for a Shiny Grey Egg. Take the egg back to the Chao Garden. Once there, hatch it by throwing it against a wall.
Return to the stage select menu. Select one of Shadow's stages to play. While playing, break apart robots to get drives. Pick up as many green and red drives as possible.
Go back to the Chao Garden with Shadow. Give the shiny grey Chao the red and green drives you gathered. Choose another level and collect more drives.
Continue giving the Chao drives until spikes appear on its head and it resembles Shadow. This usually happens after the Chao evolves and becomes an adult.