The House

The House is a character in its own right, and serves as a primary antagonist (kind of) for the entirety of Mabel.

All Seasons
The Martin House on the hill in King's County, California is an enigma; it served as protector and jailer of Mabel Martin for Mabel's entire life, keeping her safe from the abuse of her family and her own self-injury but nonetheless trapping her in its attempts to keep her safe. It is not benevolent, but it has saved both Girls with Subzero Chill from death (or, you know, re-death) on numerous occasions.

It can apparently move between worlds, and serves as a form of gateway or liminal space between our reality and the Underhill. The Martins have apparently always owned the House, or at least they did before Sally Martin left it to Anna Limón rather than her grandchild.

It has eaten Veratrine while she was still living along with Brandon's dead body. It has attempted to eat both the Girls on several occasions, because we eat what we love. It can be compared to several famous haunted places, including the houses of House of Leaves, White is For Witching, faery tales, and Coraline.

The House is one of the coolest settings of a horror story that the author has ever read about, and its existence as a real place in Infinity Mirror scares the author every day.