Kirigiri's Maternal Grandfather

You surmised correctly. The one buried in the courtyard is Kirigiri Kyōko’s maternal grandfather, his name was Tōhachirō Uzuchi, he was a talented old man who had a mastery in the 8 different traditional Japanese arts: Kendō, Jūdō, Kyūdō, Aikidō, Iaido, Naginata, and Shodō, and was also a loyal supporter of the Kirigiri family."