Age: Unknown
Height: 6.1
Birthdate: Unknown
Birthplace: Unknown
Bloodtype: Unknown
A woman named Lucrecia gave birth to a child. His name was Sephiroth!! Lucrecia never got to hold Sephiroth once. 5 years ago, before Final Fantasy 7 started, Sephiroth and Cloud headed up to the Mako reacter that is in the moutains of Nibelheim( Tifa was with them too.)
As they got to the center of the reacter, Sephiroth noticed these deformed bodies of these monsterous beings. He told Cloud that they were once human. Sephiroth realized that he had always been somewhat different from everyone. He wanted to uncover the truth about his past.
So Sephiroth was searching threw the library in the Shinra Mansion, reading every single book until he found the right information. He learned that Jenova was his mother, and that he was Cetra, and that the number of Cetra shrank, and was nearly extinct.
After what Sephiroth learned drove him mad. He destroyed Nibelheim and slaughtered everyone. Later on, Sephiroth formulated a plan, by using the Black Materia and summoning meteor to crash into the planet, destroying it, then he would merge with the planet's energy, and become one with the planet.
The Truth about Sephiroth: The truth is, Sephiroth is not really Cetra, he was human, but he was killed by Cloud. After Sephiroth nearly killed Tifa and Zack, Cloud threw Sephiroth into the dephs. The Sephiroth you talk to in the game is a clone.