This is from Wikipedia. Read. (SPOILER)Lucrecia Crescent: Sephiroth's biological mother. A young female scientist who served as an assistant to Grimoire Valentine, Vincent Valentine's father, having been dispatched to him by Shin-Ra. Her own unchecked ambitions into scientific study results in Grimoire's death, as he dies while saving her. Unable to face Vincent with the guilt she felt over his father's death, she severed ties with him.

She later became an assistant to Professor Hojo during the Jenova Project. Although Vincent, who was assigned to montior the project and guard the scientists, fell in love with Lucrecia and attempted to court her, she still felt too guilty over the death of his father to return his affection. Instead, she entered a relationship with Hojo, who — along with Professor Gast — used both her and their unborn child as the subject of an experiment, attempting to achieve the Jenova Project's goal. Thereafter, Lucrecia began receiving visions of the horrors her unborn son would commit, and in an untimely encounter, discovered Vincent shot by Hojo. After his body was placed into a large tube, Lucrecia exposed him to a mysterious form of stagnated mako called "G Substance," causing the WEAPON known as "Chaos" to be absorbed into his body. Though his life was saved, he was left in a feral madness. Lucrecia then used an ancient materia that can control Chaos to calm Vincent.

Disgusted with all that she had done and the changes Jenova's cells had wrought in her own body, she attempted to kill herself. However, she was unable to easily die because of the presence of Jenova's cells in her body, and, thus, she isolated herself in a cave located in a remote location. She is later discovered by Vincent during the time frame of Final Fantasy VII, willingly encased in a large mako crystal.