All manifestations of Sephiroth seen throughout the game, excluding flashback sequences and his real body at the Northern Crater and Core of the Planet, are Jenova's entire body under his control, shapeshifted to take on his appearance. The Final Fantasy VII Ultimania Omega Guide, littered with information directly from the creators of Final Fantasy VII, stated that Sephiroth "was not content to become a mere puppet and assumed possession of her mind and will." Despite some initial debate over Jenova's influence over Sephiroth and vice versa, this confirms that Sephiroth had taken control of Jenova's body to the extent that, in essence, he was Jenova.

Sephiroth appears through Jenova at regular intervals, until the party confronts him at the Northern Crater. At this time, Sephiroth has absorbed most of Jenova's remaining cells, so he appears to the party by way of astral projection. It is also at this time that Sephiroth's true body is seen preserved in crystallized Mako. In the game's two penultimate battles, Sephiroth takes on two other forms as detailed below. Cloud's final confrontation with Sephiroth is purely mental/spiritual and takes place in the Lifestream, a final battle of wills. Sephiroth is defeated, and his soul dissolves back into the Lifestream.

In Final Fantasy VII: Advent Children, however, it is revealed that Sephiroth avoided dissolution and is eventually revived when Kadaj absorbs Jenova's cells, allowing Sephiroth to shapeshift Kadaj's body into his own image. In this sense, Sephiroth is still dead, but as Jenova's cells still remain in existence in some way, he is still able to exert influence over them and act through them. This gives him the potential to be reborn again at a later date, if another source of Jenova cells could be found.

In the On the Way to a Smile short story, Case of the Lifestream, Sephiroth manages to sustain himself in the Lifestream by focusing on his hatred of Cloud, allowing him to maintain his sense of self. Sephiroth also believes that as long as the people living on the planet, and especially Cloud, remember him, he can continue to endure and be revived by their memories of him.

Bizarro Sephiroth
Birth of a God

Bizarro Sephiroth is the first form of Sephiroth that Cloud and his party face at the end of the original game. Although many explanations abound, it is sometimes assumed that this is a cocoon state that Sephiroth uses to prepare for his ultimate form.

Bizarro Sephiroth's Stats:

Japanese Name: リバース・セフィロス
Location: Northern Crater
Level: 61   HP: 60000   MP: 400
Attack: 210   Defense: 200   Magic Power: 144
Magic Defense: 200   Dexterity:   100   Evade: 1

Attacks: Sephiroth Shock, Fire 3, Ice 3, Bolt 3, Demi 3, Quake 3, Aurora Fence, Stigma, Heartless Angel, Bizarro Energy, Slow, Stop
Immune: Gravity, Everything
Absorbs: Fire & Earth, Ice & Lightning
Protected Against: Death, Sleep, Poison, Confusion, Silence, Frog, Small, Slow Num, Petrify, Death Sentence, Manipulate, Berserk, Slow, Stop, Paralyze, Darkness
Other Information: The Party cannot escape