Aerith was the last of the Cetra, a group of people closely attuned to the Planet. A half-Cetra, she was the daughter of Professor Gast and Ifalna. The Shinra believed she held the secret to the Cetra's Promised Land -- but Aerith also held the key to saving the Planet from Sephiroth, for she was the only one capable of activating the White Materia, summoning Holy to counter the effects of Meteor. Aerith died while praying for Holy in the Forgotten City of the Ancients, by the blade of Sephiroth's Masamune. Her death enraged Cloud, and continued to haunt him despite his defeat of Sephiroth at the end of FFVII.
Aerith returned in Advent Children, assisting Cloud from the afterlife, reassuring him of his guiltlessness in her death. She also provided the rain that healed the Geostigma disease that had infected Cloud, Rufus and many children; and saved Cloud after he was shot by Yazoo. She provided comfort in her departure, walking through her church doors, framed by light.