The Sister Ray, originally called the Mako Cannon, is a huge weapon built by the Shinra Electric Power Company in Final Fantasy VII. Its power is originally run by Huge Materia, though later is it is moved to Midgar so that it can be powered by the city's Mako reserves. The cannon's blasts of energy are powerful enough to destroy entire cities or Weapons, and can cover massive ranges while retaining their strength. The weapon's name is possibly a reference to the Velvet Underground song of the same name.
The cannon was originally installed in Junon some time before the start of Final Fantasy VII. In Crisis Core -Final Fantasy VII-, it appears in a flashback as a virtual battle ground for Sephiroth, Angeal Hewley and Genesis Rhapsodos. The battle is intense enough that most of the cannon is sliced off into the sea. However, it's just a simulation, so no actual damage is done.
In Final Fantasy VII the cannon can be seen as the party first travels through the city, but it is not used until after the emergence of Weapons following Sephiroth's summoning of Meteor. When Tifa and Barret are kidnapped by Shinra to be used as a scapegoat for the crisis the Planet is embroiled in, Tifa is thrown into a gas chamber to be executed. The plans are ruined when the Sapphire Weapon charges at Junon from the sea.
The first firing of the cannon is ineffective, and the Weapon's counter-attack blasts a hole in the fortress's armor. The Weapon charges a second blast, but the second firing of the cannon at point-blank range destroys it. Tifa escapes the gas chamber through the hole the Weapon blasted and runs onto the cannon's barrel. After a brief slap-fight with the Shinra executive Scarlet, Tifa escapes onto the Highwind.
After the near-disaster in Junon, the Sister Ray is moved to the capital city of Midgar. At Midgar its power is boosted by the combined charging power of the city's entire electrical power grid; the Mako Reactors around the city are hooked up to the cannon, creating a weapon of incredible power. Specially designed to destroy the force field around Sephiroth's base in Northern Cave within the North Crater, President Rufus Shinra puts the hopes of the survival of the Planet - and his company - on the Sister Ray.
A second Weapon appears out of the ocean, the Diamond Weapon; it comes out of the seas below the Forgotten Capital and makes a beeline for Midgar. Despite the party's attempts to defeat it, the Weapon continues its advance towards Midgar. The Sister Ray is fired with such force the windows of Shinra Headquarters shatter and parts of the cannon itself dislodges and falls to the ground. With one blast, the Sister Ray fatally wounded the weapon and destroyed Sephiroth's barrier.
However, with Diamond Weapon's final attack destroying the top floors of the Shinra Building, with Rufus presumed dead, Midgar descends into chaos and various Shinra factions fighting for control while Professor Hojo takes position at the Sister Ray's controls. Hojo's scheme is to fire the cannon one last time to give his son, Sephiroth, more power at the potential cost of Midgar from the system overload. The plan is foiled when Hojo is killed by the player party.
The Sister Ray is destroyed along with most of the Midgar in the cataclysm of Meteor falling on the city and its ruins can be seen in Final Fantasy VII: Advent Children. In the opening movie of Dirge of Cerberus -Final Fantasy VII-, just before the destruction, Vincent Valentine spots Hojo once again at the controls of the cannon. Though he thinks it to be just an illusion, this foreshadows the game's events.
- The cannon may have been named after a song by The Velvet Underground, featured on their White Light/White Heat album.
- The Sister Ray is one of the most powerful weapons ever built by man in the entire series of Final Fantasy. Only the Zeus Cannon from Final Fantasy: The Spirits Within, Vegnagun from Final Fantasy X-2, and the Cannon Superstructure mounted on the Sky Fortress Bahamut in Final Fantasy XII may be equally or more powerful.
- The cannon is not actually aimed at the Northern Cave. Although in an FMV it is seen to land a direct hit on the location, if the player aims the Highwind the same direction as the Sister Ray and travels in a straight line, not only will they miss the northern crater, but actually miss the north continent entirely. What's more, with the distance shot from, assuming Sister Ray is a straight-line energy weapon, it should have sent the blast off into space due to the curvature of the Planet.
- Scarlet tells Rufus the "Mako cannon" has been renamed "Sister Ray" after being moved to Midgar from Junon. However, while running down the cannon's barrel at Junon, the name "Sister Ray" has already been imprinted on it.
- When standing atop the cannon, the location name is displayed as "Canon".
- The Sister Ray is listed as one of the possible answers for a question in the Brain Blast mini-game in Final Fantasy XIII-2.
- An early concept artwork for Tiny Bronco has the words Sister Ray painted on the plane.