Daimon

Daimon is a word used mostly in Alliance territory to describe the Carnal alter ego of a Sinner. While those leaning towards the Roman belief system think of Sinners as a union between the daimonic entity merged with the immortal human soul, the Ally mindset is looking upon these parts of a being as separate entities. The Daimon is considered a kind of spirit with a desire and influence of its own, even though not sentient, the human possessed by it needs to learn how to keep it in control and use it to their advantage either than becoming one with it. This influence is seen as a tool or weapon that needs to be controlled and channeled to harness its powers, but that the Possessed needs to focus on to control at all cost. Failure to do so is seen as a weakness of character and frowned upon. In general society, Sinners are forced to hide their 'affliction', while being carefully surveilled by the authorities. 
One in a hundred is born this way, so it is by no means a rare phenomenon - but in Alliance territory all Sinners need to be registered, officially schooled and trained and then brought into servitude of the state. They are even marked with a magical seal containing all information about their Daimon type, nature and their stigma - that's why Sinners are referred to as 'Marked Ones' in Ally Tongue.

Quotes:

“Woah, did you see that Daimon!? It must be a tough challenge to wield it.”

Overheard in an fighting pit in Milano.

“They have a strong Daimon, for sure. I’m just not convinced they’re in control… looks more like a rough ride on a bucking mule."  "Indeed. Could become a problem. We should keep our eye on that one, brothers.”

Conversation between two Grey officers observing military training of new Possessed recruits.

Manifestation

The Daimon of a person is invisible to the naked eye, as they are prying upon the soul they are bound to from deep within the Netherrealms. Every sinner can evoke their Daimon or Ego, however, resulting in effects corelating with the Daimon's Aether resonance. A Daimon leaning towards Darkness would probably manifest as a thick, bottomless shadow of the person evoking them, a Mind related effect could be that the person or their surroundings suddenly seem wet or blurred etc.
All Carnals can call upon their Daimon to merge with them in a form of natural possession, and grow immensely powerful for a short period of time. This transformation yields bizarre and grotesque results based on the Carnal and Aether resonance of the Daimon. The only exception are Witches, who for some reason do not seem to be able to visually transform and merge with their Daimon this way. It is the but of many jokes and/or conspiracies among other Sinners.
But in any case, the Daimon is always there. Visible or not. The Aura of a person is reminiscent of their immortal soul, shrouded in the protective - or invasive - power of their Daimon. It has a will of its own and influences the carrier's personality greatly. Depending on the person's relationship with their Daimon, they can feel like a guardian, a protective entity, or a parasite, a symbiosis or even a threat for their mental balance. The Daimon has needs and will demand things in return for lending their power, big or small, frequently or rarely. And at the end of the day, each time the wielder borrows the Daimon's power, the world will retaliate by pouring Anguish into their soul. So a price has to be paid to both, the entity and the universe and every Sinner deals with that predicament in a different way.

Localization

One out of 100 people on the world is born with an inseverable connection to an immortal spirit - so Sinners aren't *that'*rare and a world-wide phenomenon.
Romans admire and strive to fully embrace their EGO and thus become stronger as an entity; convergence on the edge of consumption in some cases, even. Every Sinner wants to ascend to greatness by the side of Samael which leads to a sword edge’s dance on the brink of fully succumbing to their Sin. They hold the Inner Worlds high as they believe their savior to dwell there and can relate to dual beings and spirits as kin.
The Grey strive for control and scientific progress; admiring harnessed supernatural strength that comes from a powerful Daimon for the mind that controls it. They view it as both, high risk and high reward. They feel like they must never forget they are human and are only attached to the Carnal through circumstances, viewing all otherworldly beings as fascinating but alien creatures which must be carefully studied and bound in servitude.

Terminology Matters:

The two major rivaling factions have distinctly different terminology for one and the same concept. This is rooted in a cultural difference and understanding of the phenomenon that is a Sinner. 
While Alliance scientists cannot deny the influence of the Carnal on a person’s personality, they see it more as a condition or affliction that needs to be treated than a Sin that taints or exalts the soul like the Romans do. Here's a list with the most important terms:

• ‘Possessed’ or 'Marked One' instead of ‘Sinner’
• ‘Carnal’ instead of ‘Sin’ 
• ‘Daimon’ instead of ‘EGO’
• 'Affliction' instead of 'Confession'
• 'Anguish' instead of 'Angeli Rosso'

The Crimson terms are more wide-spread in most other areas of the known world, mostly because the Roman Empire is about two centuries older than the Alliance, and thus, has had more influence on the languages of the world. but lately it seems that their influence is fading.