Taking care of a chao with Sonic, Tails or Knuckles will turn it into a Hero chao because those three are Hero-side characters. Taking care of it with Dark characters (Shadow, Rouge, Eggman) will help it turn into a Dark chao. To keep the chao's alignment neutral, take care of it with characters from both sides. If it starts looking like it's going towards Hero (turning white, wings look more angelic, growing two small nubs on its head), pet/feed/play with it using a Dark character until it goes back to neutral. If it starts looking like it's going Dark (turning darker/black, wings look bat-like, headpiece goes wavy), pet it with a Hero character. It you keep it balanced, it will evolve into a Neutral chao.
~ Crazy C Lea