In order to make a Sonic Chao you will need to make what is called a Neutral Run Run Chao. To make a Chao look like Sonic you will need to prioritize raising its run stat a lot more than the others. The tricky part will be keeping your Chao in neutral alignment. This means you'll have to raise it using both Hero and Dark characters equally. If you use Hero characters too much you'll notice the Chao will start to turn white and gain these two small flaps on the top of its head. This means the Chao is starting to align to be a Hero Chao which is not what we want. So in this case start using Dark characters to raise the Chao. But if you use them too much the Chao will start to turn black and its wings will start to look like demon wings. Give it a while and your Chao eventually evolve into a neutral run Chao. A Chao's second evolution is more of a gradual transformation. Eventually you'll start to notice its spikes bend downward and if you use a regular two-tone Chao you'll start to see it turn blue.