Follow Fate into battle!
Alright. This is non-ero. But it is still really damn dark.
Well lets just describe Nanoha for a bit.
The first series (Mahou Shoujo Lyrical Nanoha) was rather cute at the beginning before sinking into some more serious issues (mostly based around Fate). Then came Nanoha A’s which featured a much more complex story and characters and seemingly expanded on what Nanoha introduced.
Nanoha StrikerS the characters had grown up and ten million other characters were introduced which LOOKED damn interesting but were not really looked into (mostly the well equiped beautiful ensemble known as the Numbers).
Nanoha Betrayers is a doujin that increases the seriousness ten-fold.
