Spirits are the inhabitants of the spirit plane, which collided with the world roughly 3000 years ago, bringing magic to humanity. Magic is taken or given by spirits, and spirits mainly depend on humans to increase their power.
Spirits in physical form can appear as anything. It depends on their aspect, but also on their will and personality. Some spirits are highly intelligent, while others are no better than animals, and will appear and behave as such.
Spirits take on aspects that relate to their experience and surroundings, giving rise to nature spirits, fire spirits, or more obscurely, spirits that relate to emotions or types of objects. A spirit can alter its aspect over time, which is how the first spirits adapted to their surroundings to become fire spirits and other types of spirits in the first place. Spirits also have weaknesses relating to their aspects.
A normal human can go an entire life without noticing the spirits in his surroundings, except through the impact they may have via vessels.
Some humans, however, are born with a talent for peering into the spirit world. They might be able to feel there is “something” inhabiting the cat a vessel is traveling with. They can train this sixth sense by dulling their other senses and concentrating only on sensing spirits. With enough training and/or talent, a human could be able to spot spirit energy within a vessel, be it a sparked, pacter, binder or devourer and even pin-point the spirit’s aspect.
To peer directly into the spirit world is much like lighting a candle in a dark room. The more in touch with the spirit world, the bigger the candlelight. But even with the most sophisticated spiritual sense, a human won’t see farther than, say, a 20 meter radius around himself.
Spirits and their ability to look into the mortal world is pretty much the same, except it’s not linked to birth or training, but rather their power. Thus all spirits have a slight sight into the mortal world and they usually use that to find areas of their liking, and perhaps within them find inspiration for their spiritual shape.
When a human starts a pacting ritual, he paints a circle of runes and then infuses them. This will make the runes beam like a beacon in the spirit world and spirits from far away will come to investigate. Depending on the runes used, different spirits are attracted. There are some general runes that will attract spirits of all sorts and other runes that will attract very specific kinds of spirits. Some runes can also ward against undesired spirits.