Mummy

In nearly all routes, the mummy is the reanimated corpse of Naomi's mother. She is an ally in most route, and an antagonist in the Mother & Father Ending and Monster Curse Ending.
In the Burned Route, the mummy is both Naomi in disguise as well as her mother's dead body.
In the Shadows Ending, the mummy is the reanimated corpse of Naomi.
In the Epilogues, it reveals the Mummy was just the mother in disguise.