Professor Hojo is Sephiroth's biological father.

He is responsible for the harm done to Vincent, Cloud and Sephiroth.

He experimented on Vincent eventually sealing him away. Then he turned to injecting his Jenova cells into Lucrecia, the mother of his child, and the mother of Sephiroth.

From Final Fantasy 7, and Wikipedia article on Final Fantasy 7 supporting characters. ([url=http://en.wikipedia.org/wiki/Characters_of_Final_Fantasy_VII#Supporting_cast)]http://en.wikipedia.org/wiki/Characters_of_Final_Fantasy_VII#Supporting_cast)[/url]