A sidequest where you work with one Herbert West, a rogue scientist of Shinra, to perfect his Reanimation Process. It doesn't quite work, and Aerith soul stays in the life stream, however her body is in fact animated.

Cloud will take it on dates, tell her she's pretty, and deny that this walking zombified monstrosity is clearly not Aerith. Entirely new dialogues will come up in certain fights if you have Reanimated Aerith in your party, where the villains kind of start pitying cloud, who gets very upset whenever it is pointed out the Aerith he travels with is really a monstrosity.

Come games end, when the soul of Aerith calls out the Life Stream to stop Meteor, her spirit can be seen shedding a tear as she looks down on Cloud celebrating victory with her reanimated corpse.