Cloud is an ex-member of Shinra's elite army known as Soldier, but now that he has left he is a mercenary for hire. He joins up with a rebel group known as Avalanche and helps them in the fight against the Shinra Corporation. His first task in the game is to blow up one of the mako reactors with Avalanche in Midgar.
Since Cloud is the main character in the game, he will be in your party for almost all of the game. Due to his strength it's not something to complain about. He is very skilled with a sword and is a powerful character to have on your team. Cloud is the main focus of most of the events in the game.