Valkyrie Drive is a Japanese media franchise that consists of three projects that revolve around girls who have been infected with a mysterious virus that allows them to transform into weapons.
Valkyrie Drive (, Varukiri Doraivu) is a Japanese media franchise created by Marvelous, which was announced at the AnimeJapan convention in March 2015. The franchise consists of three projects; Mermaid (, Mameido), an anime television series produced by Arms Corporation, which aired in Japan between October and December 2015; Bhikkhuni (, Bikuni), a PlayStation Vita game released on December 10, 2015 in Japan with a Western release in 2016 and a Microsoft Windows version in 2017; and Siren (, Seiren), a social game for iOS and Android devices released in December 2015. Each of the series revolves around girls who have been infected with a mysterious virus known as the A Virus (Armed Virus). These girls are divided into two classes; Extars (, Ekusuta), who can transform into weapons when sexually aroused, and Liberators (, Ribureita), who have the power to wield an Extar's weapon form, known as Liberator Arms, through a process known as Drive. 
