Ezaki Kozue

Kozue lives together with grandfather in the caretaker room.
She is an elder sister with an attractive body, her smile is never rooted out and so she is the target of many a schoolboy's yearning. However, when getting angry, she is scary instead.
There is a paragraph which indicates that she is aware of Yui beeing a protective household deity.
Her past is wrapped in mystery, and she disappears and reappears at her liking, a mysterious person indeed and obvious not quite human herself.