Byakuya Ishigami

Senkuu's father. Japanese. Formerly a lecturer at Hirosue University. Born on October 8th. Blood type O. Unlike Senkuu, he enjoys silly things and often makes romantic and poetic statements, acting as a mood maker. Although he is not biologically related to Senkuu, he has deep affection for his adopted son. Upon learning of Senkuu's interest in scientific knowledge, he sold his car to purchase various experimental tools and gifted them to Senkuu.

In the past, he took the astronaut selection exam, which was his dream, but failed the clothed swimming test due to his poor swimming skills. Inspired by the electrode Spartan suit that Senkuu made for him, he overcame his swimming weakness and passed the JAXA astronaut candidate selection exam held after ten years, being chosen as an astronaut for his mood-making abilities. Five years later, he went to the ISS International Space Station on the Soyuz, but three days later, after witnessing a mysterious light originating from South America, all contact with the ground was lost. From the footage of a fixed-point camera barely connected to the internet, he learned that all humans on the ground had turned to stone. Returning to Earth to save humanity, he witnessed shattered stone statues like hermits on a nearly uninhabited island near Japan, the farthest place from the source of the petrification beam, and accepted the reality that they were the last six humans.

Afterward, he lived with his companions on the uninhabited island, leaving behind the "Hundred Stories" as a wisdom bag for Senkuu and their descendants, who would eventually revive. Initially, he intended to create the Hundred Stories in English, but struggled with writing a novel in a non-native language, so it was created in Japanese with advice from Lillian. At this point, only Byakuya and Lillian remained of the last six humans, so their descendants used Japanese. The last of the Hundred Stories conveyed a message to Senkuu, detailing their subsequent lives and encouraging Senkuu. There is a depiction of him watching over Lillian, suggesting he lived the longest among his companions, collecting platinum for decades for the eventual revival of Senkuu and others. There is a scene where he collapses while collecting platinum in a river, but it is unclear if he died at that time. Whether he and Lillian became a couple is not specified in the story.

He came up with the idea of making a record by cutting out the bottom of a glass bottle, recording a message for the eventually revived Senkuu and Lillian's singing voice, and leaving it inside a gravestone. According to Senkuu, Byakuya himself was not particularly knowledgeable about science, and it was likely his fellow astronauts who made devices like the record.