Sephiroth is not quite human. His genetic momther was Jenova "the Calamity of the skies". He found out that he was not human on a mission to check out the Mako reactor in Nibelhiem Cloud's home town. The Shinra Inc. were making materia but with the materia they put soldiers in the mako changing them into horrible monsters. Then Cloud said to Sephiroth "what do you mean? you aren't normal?". Sephiroth then began to think about it. He disappeared then was found in the basement of the Shinra Mansion. He began reading the books in the library. Cloud can to see him. Sephiroth was laughing, he said that he was an anicent and that his destiny was to rule the world with his mother. He then got up and said "I am going to see my mother".

Sephiroth burnt down Nibelheim and killed those that tried to stop him. He went for the reactor. Tifa's father tried to stop him but was killed for his efforts.

SPOILERS!!!!!! THIS IS WHAT REALLY HAPPENED THAT DAY!!!!!!

Tif tried to kill Sephioth but he injured her and she fell down the stairs. Sephiroth went into the room labeled Jenova. Zack fallowed Sephiroth. Sephiroth Through Zack out of the room and came out carring Jenova's head. Cloud took Zacks sword and moved Tifa out of the way and presued Sephiroth. Cloud attacked Sephiroth but he was impaled on Sephiroth sword. Sephiroth then started to lift his sword up with Cloud on it. Cloud grabbed Sephiroth's Sword and through Sephiroth into the reactor's core.

Hojo cloned Sephiroth after he went missing. Sephiroth was labeled as number 1.
The First Sephiroth was Hojo's son. Hojo's wife Lucrecia bore Sephiroth, however she never got to hold him or she him. Sephiroth was told his mother was Jenova and that she died giving birth to him.