Reunion Theory

The Reunion Theory is a term in the Compilation of Final Fantasy VII. It is a scientific theory proposed by Professor Hojo that Jenova cells, once separated, will seek to gather together and return to the main body of the creature, influencing the mind of creatures they have been implanted in to do so.

Final Fantasy VII
Hojo put his Reunion Theory to the test in the aftermath of the Nibelheim Incident. The survivors of Nibelheim, including Cloud Strife and Zack Fair, are rounded up and injected with Sephiroth cells, Sephiroth have been injected with and merging with Jenova cells while in the womb. The subjects are then exposed to Mako in a procedure similar to the augmentations done to SOLDIER members. However, as the Nibelheim survivors do not have the mental or physical strength to keep their sense of self, they become mindless puppets devoted to Jenova and Sephiroth called Sephiroth Clones. Hojo tattoos the Sephiroth Clones with numbers to keep track of them. Zack, being a SOLDIER, keeps his own mind, while Cloud develops Mako poisoning. Hojo labels the two as failures and keeps them prisoner in Shinra Manor for years.

During the events of Final Fantasy VII, Sephiroth's body at the Northern Crater sends out a call for the Jenova Reunion. The main body of Jenova kept at Shinra Headquarters awakens, escapes and takes on Sephiroth's form. It attacks Shinra, and in the chaos Cloud and the members of AVALANCHE imprisoned in the building are freed. Recognizing Sephiroth's Masamune, Cloud and AVALANCHE pursue the shapeshifted Jenova across the planet to settle Cloud's grudge with him. Unbeknownst to them, the reason Cloud is driven to chase Sephiroth is due to the call for the Reunion influencing his thoughts. At this time the other Sephiroth Clones begin to move, some seeking the Black Materia Sephiroth needs for his plans.

Eventually, Cloud and the other Sephiroth Clones come together at the Whirlwind Maze, where the party witnesses Jenova in Sephiroth's form kill most of the clones, sending them into the Lifestream below, and then transforms into Jenova∙DEATH and attacks the party. The party defeats Jenova and continue deeper into the area. Eventually they meet Hojo, Rufus Shinra, and Scarlet. Hojo reveals his Reunion Theory to them, explaining he expected the Sephiroth Clones to meet at Midgar where Jenova's body was being held, but Sephiroth's powerful will overrode the Reunion instinct and brought the Clones to where he was instead. With the arrival of all the Sephiroth Clones at Sephiroth's body, Hojo's experiment was a success and the Reunion Theory is proven.

Final Fantasy VII: Advent Children
The Remnants of Sephiroth are seeking Jenova's remains, recovered and hidden by Shinra. Kadaj tells Rufus Shinra that the plague Geostigma that has infected the planet are Jenova's legacy, and Vincent Valentine later tells Cloud that it is caused by Sephiroth's cells corrupting the Lifestream and infecting the body. In a sense, this makes them similar to Sephiroth clones. The goal of the Remnants is to find Jenova's cells to bring about Sephiroth's rebirth - as they are formed from pure spiritual energy, they are not complete copies of Sephiroth, and having no Jenova cells, he cannot exert control over them directly, they are driven by their instincts to locate Jenova, who they consider their mother. However, without Jenova cells they have no Reunion instinct and cannot find her themselves.

Loz and Yazoo gather various children, including Denzel, that have been infected with Geostigma from Edge and take them to the Forgotten City. There Kadaj activates some sort of power that awakens the Reunion instinct with them, giving the Remnants a way to find Jenova. The children lead the three back to Edge, specifically the Meteor Monument in the center of town, and the Remnants destroy it with Bahamut SIN, believing Jenova to be held inside. As Cloud and the Final Fantasy VII party members battle Bahamut SIN, Rufus reveals to Kadaj he has been keeping Jenova's cells under his clothing, and in the confrontation Kadaj seizes them and the remnants escape into the ruins of Midgar. As Kadaj battles Cloud, the box containing the cells is sliced open and Kadaj merges with them. Sephiroth exerts his power over Jenova's cells and shapeshifts Kadaj's form into a new body for himself, completing the second Reunion.

Dirge of Cerberus -Final Fantasy VII-
It is revealed that near the end of Final Fantasy VII, before AVALANCHE killed Hojo in Midgar, he uploaded a digital copy of his mind into the Worldwide Network. When Meteor almost struck the planet, the power outages put the network off-line, and the digital copy of Hojo was fragmented. Three years later, the World Regenesis Organization reactivates the Network, allowing the fragments of Hojo to reform and take control of Weiss the Immaculate while he is engaged in a Synaptic Net Dive. Hojo tells Vincent he considers his digital reformation and control of Weiss a sort of "neo-Reunion".