Mom, a humanoid robot constructed by Hiyori's father after her grandmother's passing, emulates the grandmother's appearance—down to incorporated dentures—to serve as a surrogate maternal figure. Her core function is providing companionship and emotional support to Hiyori, replacing the deceased grandmother to ease the child's grief.

Physically, her movement presents a stark contrast: she walks steadily faster than a snail yet slower than a rhinoceros beetle in daily routines, but can accelerate to outpace a bullet train when required. This duality of measured everyday motion and explosive high-speed capability shapes her physical identity.