endured during the series' lifetime. Both Red Entertainment and Sega co-developed most of the games until 2008, when the series went on hiatus. Sega rebooted the series as sole developer because of fan demand. Recurring elements include anime cutscenes created by notable studios including Production I.G, and music by composer Kohei Tanaka.

The series has sold over 5.8 million copies as of 2022, and garnered