Seems like it's one of those enshrined local deities and its gimmick is that it lives inside a mirror and can be kept in check/continue to bless the family etc if enough people stare back at it/give it tons of eyes to reflect back, but turns out just photos of tons of people also work. Still, sometimes it can stretch enough of its body out of the mirror to look from behind the cupboards at unsuspecting visitors. It's definitely not a coincidence that the POV character, her mother and the deity all have very similar parted bangs - it's the family's guardian deity and/or their ancestor.