Cloud and Aerith's roles in Crisis Core, a game for the PSP.
This game is basically an overview of what happened when Zack, Cloud and Sephiroth went to investigate the reactor at Mt. Nibel. It shows how Cloud and Zack became close friends, and how Cloud got his Buster Sword.
Aerith's role in the game pertains to her relationship with Zack, and how they met. She was at the age of 16 when she developed feelings for him, while he was in Midgar away from Gongaga. Five years after that, Aerith helped her mother grow and sell flowers in Midgar.