The Demi-Fiend was once a normal high school student in Tokyo along with his friends Isamu Nitta and Chiaki Hayasaka. However, when they are summoned to an abandoned hospital by their teacher Yuko Takao, she tells the Demi-Fiend that an event called the Conception is about to take place. Initiated by a man named Hikawa, the Conception will cleanse and thus destroy the world in preparation for the creation of a new one. With nothing they can do, the high school students brace for the apocalypse.
While unconscious, the Demi-Fiend has a vision of a boy inserting a creature into his body. When he wakes up, he finds that his body has been infused with demonic power and thus he has been reborn as half human, half demon -- the Demi-Fiend. The insect-like creatures known as Magatama are what he can ingest to change his affinities and stats.
Over the course of the game, the Demi-Fiend recruits and fights demons in the new Tokyo called the Vortex World. He encounters his former friends and each becomes interested in creating their own vision of the new world to come. It is up to the player to decide which of these idealogies to join and thus take part in their creation. However, the Demi-Fiend may also choose to go against the ideologies presented and forge a different path.
Digital Devil Saga
The Demi-Fiend appears as an optional boss in Digital Devil Saga and is an infamously difficult encounter. If anyone in the party is immune to an element, he will use the move Gaea Rage to instantly kill the entire party. The music that plays during the battle is the standard random encounter theme from Nocturne; perhaps an amusing hint at what the Demi-Fiend thinks of Serph's party.