Presence (気配 Kehai ) refers to the sense of people who can feel Power of Existence that allows them to perceive others. It is not a physical sense like the five senses. Presences can vary in size and idiosyncrasies, and the more Power of Existence one possesses, the bigger their presence will be.
The presences given off by both Crimson Denizens, who distort this world's phenomena in order to exist, and Flame Haze, who store Crimson Lords in their Vessels, are presences that do not exist in this world. Therefore, their presences can be sensed, and it can be determined whether a presence belongs to either a Denizen or Flame Haze. It is also possible to tell the exact person a presence belongs to if someone excels at sensing presences, or if the presence given off is very unique. Due to this, it is very important to conceal presence in order to act covertly or carry out surprise attacks, leading to there being many Unrestricted Spells and Treasure Tools that conceal presences.
Even if a presence is concealed, if in close proximity it usually becomes detectable, and in order to attack they would need to convert and release Power of Existence, thus revealing themselves. Therefore, the ability to accurately sense presence, which is released along with the gathering and conversion of Power of Existence, is a big influence in someone's strength in a battle concerning the Crimson Realm. Hence, people who can make this theory invalid, such as Tenmoku Ikko or Pheles' Inverna, are regarded with immense danger.
Large presences are mostly given off when Power of Existence has been processed by someone to do with the Crimson Realm or with the Manifestation of an Unrestricted Spell. Though infinitesimal compared to Denizens and Flame Haze, regular humans and Torches also have presences.