シスター・プリンセス
One of the running narrative themes for the main series is Immorality, which also became the key character theme and was expressed through their personalities and actions. The second game also focused on themes of war and death. The theme for the world of "Drakengard 3", as described by composer Keiichi Okabe, is "the sense of contrast". Multiple anime series have influenced the series' characters over the years, including "Neon Genesis Evangelion", "Sister Princess" and "Puella Magi Madoka Magica". The series writer, Sawako Natori, drew inspiration for her writing from shōnen manga. The original game world was designed around Celtic and Norse myths, together with Japanese-style revisionism. The team for the original game were influenced by Asian epic movies and western action-adventure films such as the 1999 remake of "The Mummy" and "Dragonheart". While developing "Nier", the team drew inspiration from the "God of War" series, while the narrative structure was inspired by the September 11 attacks and the War on Terror. The central theme of "Nier: Automata" is struggling out of a bad situation, defined by the game's staff using the Japanese word "agaku".