Here are the steps to get a Shadow chao.
1:Get a normal chao.
2:Care for it with a dark character. (Suitibly Shadow)
3:Give it a LOT of green chaos drives untill it's ball color turns purple.
4:Repeat step 2 until it grows up.
Extra:When you first create a Shadow chao, it's spikes will be green. Keep giving it green chaos drives and it will slowly turn blue,then yellow,then orange,then red. If you want it to stay green,blue,yellow,or orange, then stop giving it chaos drives when it turns that color.