To get a chao that looks like Sonic, Collect between 10-20 green animals(deer, rabbit, kangaroo) in the action stages and give them to your chao.
Be sure to feed him! After a while, your chao will start to look like sonic. After it cocoons it will look A LOT like sonic.
Hint: If you use mostly deer and kangaroos the chao won't have those silly looking rabbit ears and will have small ears kind of like Sonic's.
If you need more help, we've got more Sonic Adventure cheats and also check out all of the answers for this game
Comments for this hint