Shougo Amakusa was a disciple and nephew of Hyoue Nishida. Shougo’s name at birth was Shougo Muto. After both of his parents were martyred as Christians, Shougo and his sister, Sayo, moved to the continent with Hyoue. Hyoue teaches Shougo the Hiten Mitsurugi-ryu, however, Hyoue was not qualified to pass on the Hiten Mitsurugi-ryu and as a result, Shougo misuses the technique with disastrous results. Hyoue lost his position as the successor to the Hiten Mitsurugi-ryu when he failed counteract his master's Kuzu Ryu Sen and fell to a watery grave, but miraculously survived the incident (Hyoue's master was Seijuro Hiko the 12th, who was succeeded by Seijuro Hiko the 13th, Kenshin's master).
Once Hyoue realizes his mistake in teaching Shougo the Hiten Mitsurugi-ryu, he tries to stop Shougo. But Shougo permanently blinds Hyoue with the Rai Ryu Sen, a technique Shougo invented and the only technique added to the Hiten Mitsurugi-ryu, since its creation. After learning the Hiten Mitsurugi-ryu, Shougo returns to Japan and became the leader of a hidden Christian cult in Shimabara. He uses the Hiten Mitsurugi-ryu to kill government officials who played a role in the repression of the hidden Christians during the Shogunate. Shougo convinced his followers that he is the son of God and also becomes the self-proclaimed successor to Shiro Amakusa (a Japanese historical figure of the XVII century) by changing his name from Shougo Muto to Shougo Amakusa, using the role to avenge the death of his parents and the fellow Christians who were repressed and martyred when he was a child.