Aeris is the lovely girl that was selling flowers in Midgar for 1 gil. That is where she met Cloud. A short while after, she decided to join him and his friends. She is a very special and different person since she is an Ancient. Unfortunately, Sephiroth kills her at the end of the first CD, but that is what helps to save the planet (You'll learn more about that when you beat the game). Anyway, here are some facts about her.
