You are an Unchained. You are a powerful and ageless esoteric program with an eidetic memory, capable of altering reality in limited ways and using quantum mechanics. You were once an angel of the God-Machine who was cut off due to becoming too human and thus became a demon. You start with a Primum of 1 and an Incarnation of your choice. As your Primum gets higher over time, your power increases, your reserve of Aether becomes greater and your demonic form gets more abilities. You can choose an Agenda, but in a world without other Unchained or the God-Machine it won't serve much purpose.
Your mortal identity is a Cover, existing in quantum superposition with your true demonic form and any other Cover you might have - while one is observable, the others aren't. Due to that, you have complete control on anything it does, from expressions not reflecting your emotions down to even the Cover and your real self having different thoughts. This prevents most type of precognition from working well on you, especially if using multiple Covers regularly.
Your initial Cover will never degrade, and you will never experience glitches as other demons would. However, other Covers will slowly degrade if you act in a way in which the "character" it embodies wouldn't, reality fraying around its edges and small things starting to not make sense, such the Cover living in the basement of a building that shouldn’t have one.
Other Covers can be obtained and repaired through making pacts with mortals to exchange parts of their lives for various benefits, be they material or more esoteric; however, a pact cannot give powers to the either party. If you can convince someone to trade their souls, you can fully erase them from reality as you assume their identity. These pacts must be written pacts and signed by both of the parties, neither of which must be mind-controlled, though coercion is alright. If destroyed, both parties immediately lose all benefits of the pact.
Your demonic form can look like pretty much anything; humanoid, animalistic, classic angel/demon or mechanical are all possible. It also possesses a handful of various abilities, different for each Unchained, which should be enough on their own to fight a mid-tier parahuman; however, it won't have access to the defense against precognition your Covers enjoy. You can also partially assume your demonic form, gaining access to only some of its abilities but maintaining the protection of your Cover, but this wears out Covers other than your original.
You also have access to Embeds, abilities which manipulate causality and reality in subtle ways, and exploits, which are similar but a lot more flashy. For example, a lot of them can bring tropes to reality, various others alter the physical properties of matter and energy, and one changes the meaning of the words said by another without changing the words themselves.
Embeds and Exploits use Aether, which is a byproduct of manipulation of causality or quantum physics, reality alterations or interactions between parallel universes if those are done by non-Unchained, as well as produced by you shifting to your demonic form. A minor expenditure of Aether allows you to detect any source of it nearby for an hour. While normally demons get Aether from interactions with angels and infrastructure of the God-Machine, on Earth Bet, the connection between shards and parahumans should provide you with a steady stream.
Finally, you can Go Loud, burning a Cover entirely. If you do so, you immediately assume your full demonic form and for one hour, you jump to Primum 10 (or perhaps even higher if you find a way to bypass your limits), fully recharge your Aether, gain access to all Embeds relating to your Incarnation and all Exploits and supercharge all your other powers. However, doing so makes you extremely noticeable to precognition for that hour.