Vestige -Yaiba ni Nokoru wa Kimi no Omokage-

Kurou was reading a book in the library when he saw his older sister Shizu fall from the roof. Fortunately, she wasn’t hurt badly. Kuro and their osananajimi Ruri are happy that she’s fine and don’t ask her what happened. Later, he finds out from his classmate Wakaba that she was fighting against some mysterious men. It is because they are after him, as he possesses the elixir of immortality ‘ochimizu’, so he decides to take part in the battle as well.