Prior to the events of Samurai Shodown, Amakusa is resurrected by the dark god Ambrosia; he uses the body of Shinzo Hattori (one of Hanzo’s sons). His main plot involves him stealing the Palenke stone, which he intended to use to bring Ambrosia to the mortal world. Haohmaru prevents this from happening by defeating him. In Galford’s ending in the same game, it is shown that Amakusa isn’t truly evil, thanking Galford for freeing him from Ambrosia’s clutches. Upon his resurrection in Samurai Shodown IV, his spirit splits into two halves: one good and one evil. Amakusa seeks to reunite with his more powerful and malevolently evil other half, who was bent on dominating Shimabara and eventually the whole world, but was unsuccessful as his evil counterpart was slain by Kazuki. His continued existence after his evil half is destroyed is mostly because of his host body Shinzo.
During the events in Samurai Shodown II, he saves certain characters from Mizuki’s wrath but because of the demise of his evil half, he isn’t as powerful as he once was and is defeated, leaving only Shinzo’s body for Hanzo to claim.
He’s a mad and vengeful spirit, resurrected by Ambrosia as a being of destruction to put up a new world order. Once he is relieved of the dark god’s possession, he reverts to his living persona as a kind and merciful soul of righteous constitution. He is a narcissist by nature, which is what originally lured him to Ambrosia’s temptations.
True to the historical Christian Amakusa, he speaks of sin, mercy and salvation, especially in later appearances.