The game also has a unique panel system which governs character improvement, special abilities, and equipped weapons. Kingdom Hearts Birth by Sleep is a prequel to the series, released for the PlayStation Portable in Japan on January 9, 2010, and in North America on September 7, 2010, with additional content. The game is set ten years before the events of the first Kingdom Hearts game, revealing the origins of the villain, Xehanort. It consists of four scenarios, three of which focus on one of the game's three protagonists, Terra, Ventus, and Aqua. The game was re-released in Japan under the title Kingdom Hearts Birth by Sleep Final Mix on January 20, 2011, with the content from the English versions as well as new features, such as an additional fifth scenario. Kingdom Hearts 3D: Dream Drop Distance was released on March 29, 2012, in Japan for the Nintendo 3DS. The game focuses on Sora and Riku's Mark of Mastery exam under Yen Sid in anticipation of Xehanort's return and their subsequent conflicts with enemies from their past. In addition to similar systems inherited from Birth by Sleep, this game features "Dream Eaters" which serve as both enemies and allies. Players may collect and breed friendly Dream Eaters and train them to become more powerful. The English edition came out on July 20, 2012, in Europe while it came out on July 31, 2012, for North America. Kingdom Hearts χ: At Tokyo Game Show 2012, Square Enix announced Kingdom Hearts χ, previously known as Kingdom Hearts for PC Browsers. It is a browser game for PCs, and is only playable in Japan since July 18, 2013. It features cartoon-like 2D models and is a prequel to the series, detailing the events leading up to the Keyblade War. Kingdom Hearts: Unchained χ: An international port of Kingdom Hearts χ that was released for Android and iOS devices. Unchained χ was released in Japan on September 3, 2015, in North America on April 7, 2016, and in Europe on June 16, 2016. Later in April 2017, it was rebranded as Kingdom Hearts: Union χ, featuring an all-new story that expanded and diverged from the original. In January 2019, the game was available on the Amazon Appstore for Amazon devices. The app was rebranded once again to Kingdom Hearts: Union χ Dark Road with the release of Kingdom Hearts Dark Road. The game was shut