Final Fantasy VII
Sephiroth is the son of Hojo, a mad scientist, and Lucrecia, Vincent's love interest. Jenova cells were injected into his embryo, giving him an extraordinary amount of power. Years later, Sephiroth, the top member of Soldier ( Shinra's Army) was sent to Nibelheim with Cloud's best friend Zack, and Cloud himself, to inspect a nearby Mako reactor. On their first trip to the reactor, Sephiroth has a startling realization that he was indeed seeing his mother in the test tube at the back of the reactor. He also realizes that he was an experiment. After Sephiroth had his eppiffany, he stayed isolated in the Shinra mansion for days. Cloud never saw him the same man again.
The Burning of Nibelheim
The next week or so, Cloud woke up in the hotel to see that Zack was gone, and that his entire town was aflame. Cloud and Tifa's master, Zangan, help rescue the few survivors left. After this had been done, Cloud decided to chase down Sephiroth and Zack to the only place they would be, the Nibelheim Mako reactor. When Tifa got there, she saw that her father was dead. She picked up Sephiroth's left behind Masamune and went to take vengeance. Within seconds of her reaching him, Sephiroth removed the Masamune and swung down, knocking her all the way down the steps.
Bye Bye Nibelheim
Zack saw this extremely unchivalrous act of his former best friend, and tried to stop him, only to fail, with the fight only lasting a matter of seconds. Cloud arrives and sees that Zack and Tifa are down. This, and the fact that Sephiroth had just slashed Tifa, drove Cloud over the edge. Zack tells him to take his sword, and so he he did.
Zack vs. Sephiroth
Cloud then, in a flurry of rage, ran behind Sephiroth, who had by now already severed Jenova's head, and stabbed straight into the back, while admitting his dreams and ambitions to Sephiroth. Cloud thought that the fight was over and ran outside to check on Tifa. Sephiroth impales Cloud through the shoulder and lifts him off of the ground. Sephiroth then attempts to throw him over the edge, only to have Cloud reverse the leverage and end up tossing Sephiroth off of the catwalk. Sephiroth saw that he needed to escape with Jenova's head or his plan wouldn't work.
Sephiroth went into hiding for the next few years, and Shinra, ashamed of what he'd done to Nibelheim, erased all files and pretended that he didn't exist. Over the years, Hojo was able to obtain Jenova's body and perform multiple experiments on her, many of them involving the Cetra, aka the Ancients, and trying to unsolve the mystery of what Jenova is. Shortly after Cloud and party invaded the Shinra company, they were captured and put in prison until Shinra was ready to question Avalanche and it's affiliates. Cloudwoke in a mysterious situation. The door to the prison cell was open and their was a trail of blood that led him and the party straight to Shinra's office, where they saw the obese business man lying on his desk, the Masamune impaling him in the back, pinning him to the desk.
There are multiple sightings of Sephiroth as Cloud and the party chase him down, but Cloud doesn't get a real lead until he gets on a ship headed for another continent. Every member on board is mysteriously killed, and then Cloud finds Sephiroth on board. Sephiroth flies off leaving a battle between Cloudand a strange monster in his wake. After the ship reaches his destination, there are many children on the beach who say they saw a man cloaked in black rise out of the water and head through town. Cloud's search continues.
Cloudwould next find Sephiroth in the basement of the Shinra mansion in Nibelheim, his home town which he assumed had been burned down. He meets Sephiroth, only for Sephiroth to leave him standing there after flying straight past him, hurling the destruction materia into his stomach, and telling him about the mysterious event, the Reunion.
Cloud then went on to the Nibelheim mountain range, after finding Vincent in the old Shinra Mansion. Cloud had a hard time fighting through the enemies that Sephiroth had left him, but eventually he got out.
The Last Ancient, Down and Out
Unbeknownst to Cloud, Sephiroth and he had not met once during his journey. Sephiroth was still at the Northern crater, waiting for the black materia. He was using Jenova's cells to recreate illusions of himself to fight Cloud. Cloud later teamed up with the pilot, Cid Highwind.
Sephiroth waited for Cloud for a long time until he and his party finally reached the Cetra Ruins. It was there that they acquired the ninja materia hunter Yuffie in their party, and it was there that they found the black materia. Hope didn't seem lost for Cloud and his party. Not until Sephiroth, the one person they needed most to not have the black materia, the reason they had tried to take it in the first place, appeared and asked Cloud for it. Cloud had no control over his body, and soon found himself walking towards Sephiroth and handing it over. Cloud had just involuntarily betrayed not only Aerith, but the rest of his party, and himself.
"Thank you, Cloud..." Sephiroth said as he floated away with an evil grin.
All hope was lost for Cloud and group....unless....unless they were able to reach the home of the Cetra on the North continent. Only if Aerith's prayer would reach the white materia and the spirits of their world. If only it would....
Cloud and party went on to face many more foes until they began to reach the North crater, and they got closer and closer to Sephiroth. Sephiroth's goal would finally be complete. One thing that Cloud didn't know was that he had not met Sephiroth once since the incident at Nibelheim. He had , in fact, been waiting for Cloud and the black materia to reach him so that he could summon Meteor. Cloud reached him in due time, and was much stronger, but still definitely not strong enough to fight Sephiroth.
He finally reached him, after Sephiroth had patiently awaited for all these years. He hoped that Cloud had become much more powerful, and would provide a decent fight, if the mind control tricks didn't work on him. After he reached them, he gave Cloud a flashback that he would never forget, or remember, either. As part of his plan, he took control of Cloud after fooling him into thinking that he had never been through any of the events five years ago at Nibelheim.
He showed Cloud that all of the memories he had were really his best friend Zack's, and that he was nowhere to be found in any of the flashbacks. Cloud wandered around in the strange world looking for himself, but he had no memory. He lost himself to the fake memories, and began floating to the top of the Mako rock. He was in no position to keep going. He suddenly disappeared, and the Northern Crater was shielded by Sephiroth's energy. All of the people there at that time were suddenly sent back to a random continent that Sephiroth desired. Even the ancient protectors of the planet formed from Mako, WEAPON, was revived. There were many of them, too. Cloud was nowhere to be found when Cid Highwind and Tifa awoke, and they had to devise an extremely last minute plan, even though all hope seemed lost. Sephiroth's meteor would finally arrive.
As expected, Cloud and party came, lead like moths to a bright light, and so they were travelling through the great Northern Crater and they met up with Rufus Shinra. He assisted the party through the strange mako and materia ridden caves. They never noticed the huge WEAPONs, though they were in the deeper parts of the caverns plain as day. They eventually made it to Sephiroth's chamber and looked up. They had gone through multiple Jenova clones that he had set up for them.
However, none of Cloud's friends had enough power to stop him from what he was planning to do. The Ultimate in all of the world weren't enough to stop him. His mother had been taken as an experiment, he had discovered everything about the world as it was, and he didn't like it one bit. And so, he became more powerful with all of his hatred and his resolve, to annihalate those that stood in his way....all to easily. The srength he'd found at the edge of the world...the black materia...stopping holy...Meteor was going to be quite an easy thing to accomplish. That, and turning Cloud slightly insane over himself and an identity crisis. He could do that quite easily, as all he had to do was mix up his memories of his past.
Into the Northern Crater
Not the Hardest thing to do in the world, he thought. He had been poisoned with Mako and thus, his memory was not very admirable for a Soldier. He would finally set his plan into motion, and it wouldn't be stopped by the likes of them.
As Cloud walked into the area that Sephiroth was sleeping dormant, waiting for the black materia, he was very wary of his surroundings. Sadly, not wary enough. The Jenova that Sephiroth had sent out was going straight for him, and he and party had a hard time fighting it off.
Rufus Shinra ran for cover and so did his other companions, scared of the unknown and what these things were. After Cloud finally defeated the weak beast replica, Sephiroth let his plan begin to unfold.
"Hello, Cloud ." he said, with an evil smirk on his face, laughing on the inside as he forced memories of the fateful day back at Nibelheim. Only, Cloud was unaware of his own memories being false. After leading on the FFVII crew and they found out about his plan to become a god and use this planet as a vassel to sail space, they attempted to stop him again. By this time, however, meteor had already been set into action, and he was at the center of the earth, prepared to take in all of the lifestream and fulfill his plans. However, the former Avalanche members came to stop him moments before, and an epic fight ensued.
Sephiroth forced to reveal his final form
Cloud and co were able to defeat him in his bizzaro form, and the Holy Materia then took them to a strange place in which they would continue battling. Sephiroth, his plan almost complete, became somewhat angelic looking in form, and power, with the ability to massacre planets. However, in the end, he was outdone by Cloud's strength, and was forced to reside in his mind. However, he would then challenge Cloud to a fight, one on one, as the crater began to crumble in on itself. He took them to a place where no one could interfere, but Cloud defeated him swiftly with Omnislash...his mind resolved. The One winged Angel fell to the hands of the young man, blood covering his face. His defeat meant nothing though, as they still had to deal with Meteor...and it was all but too late.
Sephiroth has a small role in Advent Children, but is needed to push the story forward. Sephiroth returns unexpectedly in the body of another, but with the appearance of his own self. Cloud realizes this and Sephiroth sees Cloud. They engage in a sword battle and Sephiroth, at first, is winning the fight. Sephiroth asks Cloud why he has become so weak. Cloud then separates his swords into the individual swords that composes of his sword. They all go in the air and Cloud jumps to the skies where Sephiroth with a gold aura. Sephiroth is surprised to see this new technique and Cloud slashes Sephiroth, and then he does multiple slashes to Sephiroth and Sephiroth opens up his body to the sky. Sephiroth talks to Cloud for a bit and his wing surrounds him and he vanishes, being replaced by the used body of the main villain.
Kingdom Hearts I
Sephiroth does not have much of role in Kingdom Hearts I. He appears as just a secret boss who is willing to fight you. He calmly fights you in the beginning, but as you damage him, he gets serious.
Kingdom Hearts II
Sephiroth has a little bit of a role on Kingdom Hearts II. He appears as a side story and stands in a isolated crag. He stands at the edge and looks to the distance at Heartless Castle. He gets in the first place as Sora fights Heartless in the cliffs with others and attacks some Heartless to get them out of the way to get to his destination. When Sora approaches Sora, Sephiroth talks to him about Cloud and sees the Keyblade. Sephiroth asks Sora that if he defeated the Key Wielder would he become Wielder of the Keyblade, seeing that the Keyblade was the strongest blade a person could have. Sephiroth attacks Sora for some time until in the end, Sora defeats Sephiroth and knows that Sephiroth could not wield the Keyblade. He tells Sora to tell Cloud that he is waiting at the crag they were fighting on. Sora does so and Cloud appears before Sephiroth. They engage in a battle and Tifa arrives later to see them fighting. Tifa attacks Sephiroth and Cloud unleashes his true self and power and finally manages to obtain his lost will that many others have lost. Cloud and Sephiroth then vanish in a sword struggle and Tifa tells Sora that she would have to search for Cloud once again.