Shinen is the final boss of Castle of Shikigami.
It is an evil deity which planned to enter our world through the gate created in the Castle of Shikigami. He used the Girl as a vessel to interact with our world, ordering the killings which happen before the game for the ritual to summon the castle. Once the Girl is defeated, it finally manifests itself for the final battle.
Shinen appears as a skull-shaped purple ghost. It is impossible to hit him, so the only thing you can do is survive until he depletes all of his energy. He uses the regular shots in circular patterns and large blue flames, also being able to shot a huge frontal laser. He will shrink in size as the battle goes on, exploding when he is finally exhausted.