Short answers, pulled from the story.
Dissidia Final Fantasy is a fighting game developed and published by Square Enix for the PlayStation Portable. It was released in Japan on the 18th of December 2008, in North America on the 25th of August 2009, and in Australia and Europe in September 2009 as part of the Final Fantasy series's 20th anniversary campaign.
Dissidia Final Fantasy sold over 1.8 million copies worldwide. By August 2009 it had moved 910,000 copies in Japan alone, and the NPD Group ranked it the best-selling PSP game in the United States for all of 2009.
Dissidia Final Fantasy was directed by Takeshi Arakawa and produced by Tetsuya Nomura. Nomura originally conceived the game as a Kingdom Hearts spin-off featuring Disney characters while working on Kingdom Hearts II, but shifted to Final Fantasy characters after feeling uncomfortable putting Disney characters in direct combat against each other.
Dissidia Final Fantasy uses a Bravery Points system where players must steal BRV from opponents before they can deal HP damage. A filled EX Gauge allows characters to enter EX Mode and execute an EX Burst, a powerful unavoidable attack comparable to the Limit Break mechanic from the mainline series.
Terra Branford was chosen as the Final Fantasy VI hero because the development team wanted a female fighter on the side of Cosmos. Final Fantasy VI features multiple characters who could fill the protagonist role, but Terra was selected specifically to ensure gender balance among the ten warriors.
Dissidia 012 Final Fantasy launched for the PlayStation Portable in March 2011 as a prequel to the original. Later entries include Dissidia Final Fantasy NT for arcades, PlayStation 4, and Windows, and the mobile title Dissidia Final Fantasy Opera Omnia, which released in Japan in February 2017 and internationally in January 2018.