From PSU: “All three characters have a powerful form called Devil Trigger that, when used in battle, help deal tons of extra damage. Devil Trigger is limited to how big your bar is, though. With Purple Orbs, you can extend that bar, lengthening the amount of time you have to deal that extra damage in that stronger form. Scattered throughout the game are eight fragments, granting two extra bars.”
Source: N4G PC Devil May Cry 5 – Purple Orb Fragment Locations (All Devil Trigger Upgrades)