"Shin Megami Tensei: Digital Devil Saga, known in Japan as Digital Devil Saga: Avatar Tuner (Digital Devil Saga アバタール・チューナー, Dejitaru Debiru Sāga Abatāru Chūnā), is a PlayStation 2 role-playing game developed by Atlus and first released in Japan on July 15, 2004. It is a spin-off of the Shin Megami Tensei series of video games; the main difference between Digital Devil Saga and the main series is the focus on specific characters rather than summoning demons to fight alongside your party. The story to Digital Devil Saga is continued with its direct sequel Shin Megami Tensei: Digital Devil Saga 2.
Notable features of the game include the ability to shapeshift between demon and human forms in battle, and a return of the "press turn" battle system from Shin Megami Tensei III: Nocturne. A large difference between Nocturne and Digital Devil Saga is the ability to change equipped skills at will, instead of at level up. Unlike in Nocturne, unlearned skills are not lost forever, which allows for much greater character customization. The game features a "Mantra" system where Macca gained after battle can be used to download new Mantras at a saving terminal. Atma, gained by devouring enemies in battle, then allows the characters to Master these mantras, imparting new abilities."
Source: Wikipedia, "Digital_Devil_Saga," available under the CC-BY-SA License.