Genmu

Genmu (幻夢,Phantasm) are spirits used by the Arahitogami.

Overview
Genmu are sentient entities that are composed of concerted energy that is found within the Gengetsutani. At the moment a Arahitogami is a Genmu is born along side them somewhere within the world. Upon reaching a certain age they are sent out to find it and must make it submit to them. Once done, the Genmu will merge itself with them, giving them access to its power.

The relationship between a Genmu and an Arahitogami is stated to be symbiotic in nature.

Abilities
Araware (表れ,Manifestation): At any given time an Arahitogami is able to manifest their Genmu to fight beside them. However, maintaining a Genmu in this form requires large amounts of energy.

Busō (武装,Armament ): the standard application an usage of a Genmu. By partially manifesting their Genmu. The user will shape it into the form of a weapon that the user will take into battle. While often appearing as a weapon. It can also take the form of armor.

Gugen (具現,Incarnation): is an application in which the Arahitogami allows the Genmu to temporarily take over their body. In doing so, the Arahitogami takes on the appearance of the Genmu.

Isshindōtai  (一心同体,One in Mind and Body): An ability born from the bond between an Arahitogami and their Genmu. By resonating their souls with one another, the two will merge into a single entity. Once created, a new personality is created, as opposed to two minds sharing a single body. In addition to this, it is also stated the created entity is known to be greater than the sum of its parts, as it encompasses the abilities of the two.