Motomitsu and his girlfriend

Motomitsu (right), and his wife, (left).

The Master doing his job

Motomitsu and his wife impressed by The Master's cocktail.

Miwa's grandmother is the wife of Motomitsu Kurushima. They have been together for an unknown amount of time, though is concidered fairly long until she died (as shown when they went on a date together to a bar, where they had The Master's delicious cocktail). At the end of Chapter 2, Motomitsu states that Miwa Kurushima, her granddaughter, reminded him of his wife. She is deceased, and when she died remains unknown.