You're watching a game you've already played, but animated, which means you're seeing moments you thought you understood suddenly shift under a different light. The OVA doesn't exist to tell you a new story; it exists to make you feel the scenes you already lived through all over again. If you've beaten Hakuouki Shinkai, this is fanservice that actually earns the word, not new content, but a mirror held up to choices you made, animated with enough care that your memory of playing it starts bleeding into what you're watching. This is for the player who kept thinking about specific moments after the credits rolled. It's short. It knows exactly what it is.
Hakuouki Shinkai: Kaze no Shou is a single-episode OVA that came packaged with the Limited Edition of the video game of the same name. Rather than telling a standalone story, it strings together a selection of scenes taken from the early part of the game, animated by Idea Factory. The genre labels place it in action and drama.