Minashiro Soushi is basically the only one of 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 and Japan was destroyed 28 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. Though he was very close friends with Kazuki as children, due to an incident in which Kazuki ended up blinding Soushi's left eye, they have drifted apart, but find themselves once again clinging tight to each other for support when the Festum attack Tatsumiya and it is up to the two of them to defend it.
Because of his incomplete vision, Soushi cannot pilot a Fafner and fight the Festum hand-to-hand. Instead, he acts as the battle commander who connects directly into the minds of all the Fafner pilots through a super computer called the Siegfried System, and directs them in battle. All the pilots, especially Kazuki, hold to him for strength and leadership-- a burden that he shoulders with grace and stoicism.
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. Soushi also has a little sister named Tsubaki who is the island's Core and powers all the computer systems on it.