Soushi is the only one of all the children on Tatsumiya Island who knows the truth—the world has been under invasion by an alien race called the Festum for the past three decades after Japan was destroyed over 30 years ago. He is the son of Minashiro Kouzou, the commander of the secret military fortress Alvis, which is hidden in the bowels of the peaceful-seeming Tatsumiya Island. From a very young age, Soushi was bred to become the next commander of Alvis, so he has had a very isolated childhood carrying the heavy secret of the war and being told day in and day out that he must live only to protect the island.
Because of his years of virtual isolation, he is rather awkward socially, (in fact, most people find him quite unapproachable or mistakenly think him hard-hearted). Kazuki alone seems to make efforts to understand and befriend him and the two become very close over the course of the series despite an unfortunate incident between them that occurred when they were younger