Armor

In almost all routes, the suit or armor is worn or possessed by the antagonists of each route.
In the Monster Curse Ending, the armor is possessed by the ghost of Nami and Naomi's father, forced to sate Naoki's hunger.
In the Monster Bloodlines Ending, the armor is also their father, but as a monstrous ghoul that has always been a suit of armor.
In the Firefly Ending, the armor is possessed by an unnamed businessman who lusts over and assaults Nami.
In the Epilogues, it reveals the armor was just their father in disguise.