From PSU: “Devil May Cry 5 brings back secret missions hidden throughout the game. The twelve secret missions can be hard to find and completing them will reward you with a Blue Orb Fragment. These missions will challenge you in various ways, and each one is worth doing. Even if you cant complete them during a mission, you can always challenge them in the main menu after you unlock them.”
Source: N4G PC Devil May Cry 5 Secret Levels Locations – How To Find The 12 Missions