How do you keep a chao from dying in Sonic 2 player battle

When it reaches the end of its life cycle (the second transformation) it will either have a gray (death) or pink (rebirth) bubble. It is more likely to live if you treated it well, but if it is near the end of its lifespan (an age of around 5) you can save and exit chao garden, then wait for it to bubble. If the bubble is gray, reset the system so it doesn't save, and try again.