Mortals

"As long as you are alive, you have a choice. This is Ma'at. This is the right way of the world." -Abbess Pebeseth.

Free will is one of the greatest gifts of all.

This is what marks mortality - some would say it is the inevitability of death, but the wise know it is the ability to choose, freely, an unwritten fate. This is the blessing, and the curse. Supernatural creatures; the incarnations, celestials, demons, and the dead, are all tied to aspects. Aspects are philosophical concepts or narrative tropes that provide a role for any being that has them. They are ability to make narrative change, to move the world around you. Free will, on the other hand, is the ability to move yourself. Aspects let a being affect the cosmos but the cosmos affects them back in equal measure. Aspects are the theme music in the background, the genre of the story, the goal and the motivation for all beings subject to them. Virtues, motifs, ideals, domains, hollows and vices are all different types of aspect - they are choices and they are also destinies.
The only aspect a mortal is born with is simple mortality. Humans are human, dwarves are dwarfish, and catfolk are feline. Anything else they represent, stand for, or are bound to is based on a choice someone made. Different types of magic permit this choice. Cultivation places an aspect, like a virtue or ideal, into the spirit. This comes with limitations and benefits. Sorcery can place aspects on the body, a spell cast on a person might attach an aspect to them, mages have aspects temporarily while they cast spells that are suffused with them.  Choosing an aspect, championing it with mundane means or taking it into yourself with magic makes you, the mortal doing it, stronger and more effective as part of the exchange.
Mortals do not have supernatural powers, but their personality, morality, and fate are in flux. They can always make another path, reinvent themselves, change their names and their beliefs. Incarnations have to fight against the essence of what they are to change. Celestials risk destruction for turning from their appointed path. And to be undead or demonic is to lose your free choices, even as you become immortal. Some of the most powerful beings in the cosmos are compelled to play their part as actors on the stage, bound by what is written in their script. Mortals can improvise all their lines, for better or for worse.
Many powerful people are demimortal: mortals with an aspect who have not committed themselves fully to inhumanity or the equivalent. Cultivators, witches, sorcerers, immature supernaturals and people with supernatural ancestry all have nascent aspects they have not fully chosen. The affinity they have makes magic easier but relying on that aspect feeds it, makes it stronger. Before long, someone touched by an aspect has to decide if they wish abandon it and live untethered or permanently commit to something greater than themselves.
Animals are also mortal, but they are ruled more by instinct than choice. Many animals represent aspects even if they do not utilize or internalize them. These nascent aspects make good subjects for wizardly incantations or sorcerous magical components. Even though owls are no wiser than other birds and tigers have no power over iron, the owl still represents wisdom and the tiger still represents the element of metal. Demihumans share similar aspects to the an animals they resemble, either because they are related or because people believe them to be. There is little difference between truth and belief with aspects. An animal that becomes awakened to sapience can control their aspects and do magic like a demihuman or other mortal. But like any person, once they are able to make choices they must live with them.
Ultimately, it is neither good nor bad to surrender some part of your freedom for some greater blessing. Any mundane partnership, relationship or covenant does likewise. In fact, there is a metaphysical element that swearing an oath or getting married has that resembles the magical changes of taking on an aspect. But mortality is precious. If a mortal cultivates enough to leave their mortality behind, or receives enough demonic corruption to reach the point of no return, this transformation causes them to change physically. This apotheosis tends to create an incarnation, celestial or demon matching the process by which they changed. The gift of mortality is that this is a decision one can make like any other. But it is a decision difficult to take back.