Aeris (or Aerith, if you prefer) is introduced in the opening of the game, standing in front of a pipe with a Mako leak with her hands clasped in prayer. What she is praying to is a mystery, but slowly throughout the game more is revealed about her strange connection to the planet and to the lifestream, the lifeblood of the planet that Shin-ra has been harvesting as Mako energy.
Aeris is a flower girl who has lived in Midgar with her adoptive mother Elmyra for most of her life. She meets Cloud briefly as he fleas from the reactor explosion and if the player selects the right dialog, she sells him a flower. Aeris later meets Cloud again when he falls through the ceiling of the old church where Aeris grows her flowers. Aeris is initially drawn to Cloud by his resemblance to her old boyfriend Zack. She asks Cloud to be her bodyguard to help defend her against the Turks (who seek to capture her for Shin-ra) but also so she can find out more about him.
In Aeris’ adventures with Cloud, she is captured by Shin-ra. When Cloud and the others go to rescue her, they find out about Jenova, Sephiroth, and the Ancients. Aeris is one of the last Ancients, also called the Cetra, and wants to learn more about them and her connection to Sephiroth and Jenova, who may also be Ancients. When Cloud and the others escape Shin-ra and leave Midgar, she joins them in their pursuit of Sephiroth and their mission to save the planet from the evils of Shin-ra. During the journey, Aeris learns about the Cetra and what she must do to save the planet. She goes off alone to stop Sephiroth who plans on using the black materia, the ultimate destructive magic, to summon meteor. Aeris goes to the lost City of the Ancients with the white materia to pray to the planet to summon Holy, the ultimate protective magic. Cloud and the others do not know how Aeris is planning to save the planet, but Sephiroth knows. He kills Aeris in front of Cloud before anyone has a chance to act. Only later in the game do Cloud and the others discover the hope Aeris has left them with. Her prayer for Holy reached the planet before she died.
The game left many questions about Aeris unanswered, leaving much room for fan speculation. In this section I will discuss some of these remaining questions, including whether or not Aeris needed to die to save the planet and if she knew the fate that awaited her.