Fear is a by-product of the process of living.
.source
Fear is the result of a life form being subjected to stress or conditions which are directly perceived as affecting its chances of survival. All beings that require Anima experience fear when they feel their lives are threatened, even if they lack the hormonal capabilities for it, as long as they can experience emotional fear or have an instinctive, organism-wide response to danger.
.properties
Fear leaves quick-fading imprints on the local Anima resonance, scaling with the amount of fear experienced and the physical and mental complexity of the being. Devils are capable of sensing and metabolising these imprints, increasing their reflexes, senses, speed, and strength, although the fear threshold required for a Devil to empower another Devil is significantly higher than that of regular life forms. If a being lacks the ability to feel fear or any sort of organism-wide self-preservation instincts, they will leave zero imprints.