Nagasumi's Grandfather (Michishio, first name not known) is the late grandfather of Nagasumi, the husband of Nagasumi's Grandmother, father-in-law of Nagasumi's Mother and father of Nagasumi's Father.
Nagasumi's Grandfather died when Nagasumi was five, but he occasionally meets his grandson again whenever Nagasumi has a near-death experience.
Not much is known about him, except that he was from Seto and met his wife who he would occasionally take to the Bon festival. He is implied to have had a happy marriage with his wife (unlike his son's marriage) since she honors him in the first episode, and he mentions his wife "visiting" him in the manga (despite being in the afterlife).
It is implied that he did not have a good relationship with his son. In the first episode, while his wife and Nagasumi were making offerings to his shrine, his son was watching TV instead. Nagasumi also suspects that his father did not have a good childhood.
Unlike his son and daughter-in-law, but like his wife, he is very fond of his grandson. However, in the anime he tried to beg Nagasumi to stay with him in the afterlife (while lying by saying they were in heaven) while in the manga he is more passive (and bluntly tells Nagasumi they are in the underworld/Yomi).
Nagasumi always meets him in the afterlife equivalent of Shikoku, an island which is close to Seto Inland Sea (in Japanese religion, the afterlife Yomi is identical to the living world).
It is implied that he was a veteran since his wife mentions that the money she gave Nagasumi for his date came from "war/campaign funds" and images from his youth show him in a military uniform.
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