The name says what she is. Ape is fire and huci is grandmother, and the kamuy of the hearth is addressed in the terms a household uses for its oldest woman. That is not a courtesy applied from outside; it is the relationship. She is resident rather than visiting, present in the house continuously, and the one member of the kamuy world who is always there.
Her place is the hearth at the centre of the Ainu house, and the hearth is the centre in more than a floor plan. It is where food is cooked, where the household sits, where guests are received and where the seating order is fixed by who a person is. The fire is not allowed to go out casually, and a fire that has been let die is a household failure of a specific kind rather than an inconvenience.
Because she is always present, she is a witness. What is said and done in the house is done in front of her, and undertakings made at the hearth are made in her hearing. She is addressed in the ordinary voice of the household, not in a formal register kept for great occasions, and the addressing is done by whoever is there.
Old age is the point of the grandmother title rather than an incidental detail. In the Ainu account authority in the kamuy world, as in the human one, comes with having been around, and the fire has been around longer than anyone. She is the senior person present in any house on the island.