Neutral chao are just that, neutral. They arent dark, or hero. Sometimes people get confused, thinking the only neutral chao are the plain blue/lime green ones that dont have an element. This is not the case. They are just any chao thats evolved and isnt dark or hero.
Generally Neutral chao are just like every other chao. These chao were also in Sonic Adventure on Dreamcast. The most popular of the neutral chao is - by far - the Sonic chao. AKA Neutral Run + Run. After that is the NiGHTs chao, aka Neutral fly + fly. Then the Sumo chao, (Neutral Swim + Power) looks like a chao with a giant stomach.