Unlike his grandson who’s sensitive to cold, he’s sensitive to heat. He enjoyed doing rubdown and was getting senile and having bad visual perception. He was however able to remember Tatsuya’s last birthday in summer, but collapsed and died of heatstroke. His spirit lingered, constantly cooling his room, which his grandson started using, by setting the air conditioner to the strongest cooling, which was more than Tatsuya could take.
Rinne, sought by Tatsuya, makes him visible, and the old man reveals a hidden wine he bought the day Tatsuya was born that he was going to offer to him on his 20th birthday and drink with him, meaning his motive was to keep the room cool to ensure the wine would not be spoiled by the high temperature, which touches Tatsuya shedding tears, and his grandfather passes on, revealing to have been really senile as he did not realize Tatsuya was actually 16.