Spirits

"What distinguishes a spirit from a man? How do you distinguish a Man from his soul?"

— unknown scholar

Simply speaking a spirit is an immortal being living in the Spirit World.
More broadly speaking, spirits are strange and terrifying agents of universal concepts and manifestations of natural laws, and often were mortals once.

Nature:

In the Brass realms, concepts have power. There is a tangible, real power to things such as 'evil' or 'order'.
One can attune themselves to these concepts and draw tangible power from them, the ability to change warp reality and change the world to suit one's desires.
A Mortal mind however can only attune itself to those concepts to a certain degree while alive. Once they pass on however, they are able to dedicate themselves so fully that they become part of that concept for all eternity, becoming a spirit.
Spirits are thus souls imbued with the power of a universal concept or Force. This Force will shape and define them in the eternity ahead of them.

Reality a dream:
Spirits have power, that is undeniable. To a spirit, reality is akin to a dream and the more powerful a spirit is, the more it is able to lucid dream. Powerful spirits are able to shape reality to their whim and the most powerful of them all are indistinguishable from Gods.

No longer Mortal:
Spirits are sentient and have emotions but they in large parts do not have free will or the ability to freely make decisions like mortals do. They are very often unable to distinguish between good and evil and their nature does not change. Death has changed them irrevocably, a spirit might still have desires, such as harming or helping living beings, but it does not make a choice to follow these desires; they are inherent to its nature.

Grow with time:
Spirits become more powerful the more they attune themselves to their chosen concept as well as with time and dedication. For example, Malaci grow as they acquire knowledge and Nightmares grow as they cause Fear. Additionally, any Spirit can grow by consuming fragments of others of their kind. However there is a limit in how fast a spirit can 'digest' pieces of their fellows without hurting themselves. As a result spirits grow rather slowly, advancing the ranks over the course of centuries if all goes well.

Greater Spirits:
The oldest and most focused spirits are called Greater Spirits. This category includes powerful beings like Devils and Angels. They are often so powerful that their actions can shake civilizations.
Greater spirits are often resistant or outright immune to the protections and countermeasures commonly used against ordinary spirits.
Deities are also technically greater spirits, though their origins differ.

Manifestation:

Spirits mostly dwell within the domains of the Spirit world, where they linger, far away from mortal concerns.
Mosts spirits are unable to enter the mortal realms by themselves. At most they can linger at the border, watching the mortals going about their lives until something allows them to manifest.
If they are summoned or for some other reason choose to interact with a material plane, they will form a 'manifestation' or 'avatar'.
This avatar must be maintained by a constant flow of energy. If they are not sustained in some way, they will weaken and fade back to the spirit world from whence they came.

Spontaneous manifestation:
In places where the element the spirit represents is abundant, spirits are drawn to them and can enter the border between the material and immaterial. Spirits of death may manifest on battlefields, fire elementals in the middle of forest fires, Nightmares in population centers during times of unease and fear.
The conditions required for a spirit to spontaneously manifest are usually only briefly present and only allow for weak spirits to appear. The spirit then either needs to find a source of sustenance or be banished back to the spirit world.
If the forest fire ceases, the fire elemental is banished. A nightmare needs to continually frighten people in order to stay in the mortal world, etc.

Centers of power:
There are also certain rare 'hotspots' where the border between the spirit world and the material world are permanently thinned and a spirit domain leaks into the material world.
These hotspots can occur naturally, or can be created artificially with the use of rituals.
Natural examples might be a volcano on an otherwise cold world, or a magical forest.
Artificial hotspots could be something like a haunted house or the consecrated ground around a holy site.
In such natural environments a spirit can manifest of their own accord, and the nearby presence of their home realm sustains them so long as they do not leave.

Summonings:
A magic user may also artificially create a suitable environment for the spirit to manifest as part of a summoning ritual to bind a spirit as a servant. These manifestations are then sustained by the magic of their summoner and thus are able to move wherever the summoner commands them.

A temporary body:

No matter how a spirit comes into the world, if it is able to manifest, a spirit will then create a body for itself. A Fire elemental will create an avatar of flame, a rock elemental a hulking mass of stone.
These bodies are made of ectoplasm, spirit-matter from the Spirit World. The forms they assume feel real but in truth are temporary and dissolve into nothing if the force animating them is dispersed or disconnected.
Not understanding this, many Mages have summoned gold Elementals, thinking they could harvest them, only to be disappointed when their seeming wealth evaporated before their very eyes.

Destruction:

As a result of their intangible nature, spirits are immortal, the destruction of their corporeal manifestation only temporarily harming them.
Weak spirits will simply be thrown out of the Material plane as they lose cohesion and quickly reform. More powerful spirits, like Angels or Devils will however be directly sent back to their home-realm, where they will slowly and agonizingly reform. Generally, the more powerful a spirit and the harder they are to vanquish, the longer they need to reform.

Thresholds & Protections:

Spirits come in very large variety. Some are able to self manifest, able to affect the mortal world directly by themselves. Some can possess or influence mortals, without having to expend the effort to enter the world at all. Some can only be summoned, but once summoned can burn entire cities to the ground. The possibilities are many.

As a result, as a staple of many cultures, rituals and customs have been established to protect the people against undue harm from spirits.

Wards, Blessings & Charms:
It is customary that dwellings and other buildings are warded with protective charms as well as blessings by priests to ward off evil spirits and fiends. Alternatively these beings could be scared off by use of creatures like temple dogs. These measures serve to make these protected places as unpleasant as possible for fiends while not scaring away any celestials.

Spirit Festivals:
Many cultures have a customary 'Spirit Festival', where the intent is to honor the dead and the various good aligned spirits and celestials watching over mortal people. Usually such festivals incorporate sacrifices and ritualistic dances, but smaller variants of such festivals can occur as daily rituals as part of certain cultures.

Names:

A spirit is a metaphysical force, but it remains identifiable by its name. Their name changes from their mortal name, and is granted upon their conversion into a spirit.
A name is very precious to spirits and they usually guard them jealously, and for good reason. By using their name, a skilled magic user can get control over a spirit and enslave it. A mortal even simply speaking their name draws their attention. Weaker spirits will unknowingly be drawn to the ones that speak their names, easily manipulated and guided.

The connection however goes both ways, the more powerful and the closer a spirit is, the more likely it will know that you spoke its name. The connection can then be used by the spirit himself to find the one that said its name, or if they are very powerful, to immediately perceive everything around the speaker. This clairvoyance can be very long lasting as well, so it is not smart to speak the name of fell gods lightly as you are giving them knowledge on you and your surroundings whenever you do so.
In some extreme cases, speaking the wrong name lightly has even killed the speaker in question, as the spirit took offense, self-manifested and slew the speaker in quick succession.

Another use for a spirits name is in summoning rituals, to summon a specific spirit rather than one of a certain type. By knowing a spirits name, it becomes much easier to summon and control a spirit. Usually any two spirits of the same 'category' are interchangeable and thus it doesn't matter which spirit you call upon, but there is spirits that openly spread their name and wish to be summoned by mortals. Such a spirit will likely not resist being summoned, manifesting with intent to bargain with the summoner for power or information, then peacefully leaving once their services are no longer needed. These spirits are usually quite powerful indeed and could not normally be summoned if they did not wish it.