One of the greatest mysteries in Final Fantasy is the secret of Cloud's past. Is he a clone of Sephiroth? Is he even human? It's very hard to tell. Here, for your benefit, I will lay out the evidence and dispell all the mystery about who Cloud really is. Be advised that this is a definite spoiler, and if you want to save the mystery for yourself, leave now.
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Originaly, Cloud did leave Neiblhiem to join SOLDIER. However, he was found to be unfit, and so he became nothing more than a common grunt. During this period, he became friends with a SOLDIER member named Zack. As luck would have it, these two were assigned to accompany Sephiroth to the Neiblhiem reactor. Cloud, too ashamed of his failure to admit it to his friends and neighbors, kept his mask on when they arrive. He does, however, stop and talk with his mother a little bit. When they reach the reactor, it is Cloud who stays outside and guards Tifa. Inside, Sephiroth finds Jenova, and his mind slowly starts unraveling. He isolates himself in the mansion library, eventually burning the town to the ground in a rage. Zack and Cloud follow him to the reactor. Just as they arrive, Sephiroth slashes Tifa. Zack chases after him, while Cloud carries Tifa off to the side, fulfilling his childhood promise. Zack and Sephiroth battle, and Sephiroth sends him flying out of the room, mortally wounded. He leaves, carrying his sword in one hand, Jenova's severed head in the other. Zack pleads with Cloud to kill Sephiroth, so Cloud takes Zack's sword, then gives chase. When Cloud reaches Sephiroth, he stabs Cloud in the chest and hefts him into the air. Thinking Cloud dead, Sephiroth lowers him back down. Suddenly, Cloud grabs the blade of the sword, lifts Sephiroth, and flings him into the liquid Mako. (This means that the Sephiroth you've been chasing around is nothing but a clone created by Hojo.) Cloud then collapses. Some scientists find them, and place them in glass tubes so they can heal them, as well as inject them with Jenova cells. Cloud has a reaction to them, while Zack does not. During this period, Cloud and Zack were linked, which is why Cloud has some of Zack's memories, and yet sees himself in Zack's role. It also explains why he has Mako eyes. They showered him with it to speed his recovery. Eventually the two escape during feeding time. They leave Neiblehiem, and begin hitchhiking to Midgar. There, they intend to become swords-for-hire, and earn a living. However, as they approach the city, they are found by a group of Shinra soldiers, who were searching for them. Zack is promptly shot to death, while Cloud, badly injured, is left to die. However, he manages to get to Midgar, where he slowly recovers. Finally, 5 years later, he finds Tifa at the train station. She at first mistakes him for Zack, because they look and act so similar. (The result of their link during their recovery.) Finally, she convinces Cloud to take the job with AVALANCHE, and you undoubtably know the story from this point on, having played it already. There are those who beleive that a sephiroth clone living in a pipe in the slums is really Zack. I don't share that view, simply because I havn't seen any conclusive evidence to prove it.
There. That's the whole truth behind Cloud's past. Be this point you should know that he is a normal human being, that he never was in SOLDIER, and that he did kill Sephiroth.