Demons

Demons are supernatural creatures that inhabit Hell. Demons were either previously human sinners that died and were sent to Hell or similar to Charlie's case, Hellborns that are naturally demonic in nature, and thus not having been previously human.

Description
Demons come in all shapes and sizes. Their physical forms typically have characteristics associated with the human body, with the exception of having ears and sometimes lack of a nose. Demons typically have two forms, their default form, and their "Full Demon" form. They can easily shift from their default form to their "Full Demon" form with little to no effort, they also tend to gain an extra or new set of appendages such as an extra pair (or more) of eyes, horns, or simply just get enhanced versions of their default form's body parts (such as the rest of Charlie's teeth becoming pointed fangs).

The typical abilities that Demons possess are the ability of demonic transformation, different varieties of magic, and even the natural abilities of the animals their characters are based on (such as night vision, heightened senses, and hunting instincts).

Demons typically don't die easily but they all inevitably succumb to the weapons used by Exorcists. When hit by their weapons, it results in the body of the Demon getting "killed off" with the dark energy that's left in the soul to wander into inanimate objects in its surroundings, which explains why some things such as regular furniture have functioning eyes. Although they are the souls of dead sinners, Demons are shown to still consume food and the like which suggests that their bodies still need the intake in order to survive. Sinners stop aging once they are turned into demons and remain the same age as when they have died (ex: Alastor dying in his 30s will remain 30 years old for the rest of eternity in Hell). As for Hellborn Demons, they are proven to age, albeit slower than humans (as shown in The Magne Family portrait of 1875), maturity and intellect-wise but they show little to no changes when it comes to their physical forms. It has also been stated that just because they live for a long time, it does not make them immortal, suggesting that they can still die of old age or by natural causes.

Culture
There are two main groups in Demon society: The Sinners and The Hellborns.

Sinners are humans who have done bad deeds on Earth that resulted in their punishment of being sent to Hell and thus being transformed into Demons. Whereas Hellborns are "Original" Demons that are born in Hell, thus them not having had any experience in the human world or any experience as a human. Hellborn demons are able to traverse across any of the rings of Hell, whereas sinners are confined to the Pride ring of hell.

All demons govern by a hierarchy. This hierarchy consists of three levels: the Sinner demons at the bottom, Hellborn demons in the middle, and Lucifer Magne above all other Demons (even Satan and Beelzebub). However, there are exceptions to this system. For instance, a Demon can elevate their status by accumulating dark powers, which they receive by making deals, soul dealing, or even taking it by force. Additionally, different species of Hellborns have a designated standing. For example, Imps are the lowest type of Demon in spite of being Hellborn Demons, and the canine-like Hellhound Demons are demons for hire. Unique hierarchical levels separate from the general hierarchy also exist, such as the Ars Goetia-inspired Goetia Demons.

There is a specific type of stronger than average Demon referred to as Overlords, which counts as virtual Gods of the underworld. These Demons possess enhanced abilities and frighteningly destructive powers that no mere Demon can match, granting them a high place in Hell's hierarchy. They usually obtain/enhance their powers by making deals with other Demons as well as dominating them in battle and taking in their dark energy.

Demons have their own languages similar to how humans have a wide variety of languages, as revealed when Charlie is said to be fluent in every Demon language she has learned while growing up. Although they have yet to show anyone speaking in this language, as of now the characters have only been shown to speak human languages such as English (for the most part) and Spanish (by Vaggie).

The Demon civilization is not that different from that of human civilization as they also have laws, facilities such as shops, news stations, bars, etc. and a hierarchy that's to be followed and respected. This also applies for their forms of technology as it is also not that far off from human technology, but with their own twists to it.

Most Demons hold little to no value for the preciousness of life. Being part of a society wherein many individuals share the same traits, demons have come to objectify any and all life forms, allies and enemies alike. As such, they tend to treat each other and themselves as disposable commodities.

While there has never been mention of Demons practicing any form of spiritualism or religion, the existence of a sign that says "Satan Bless" that can be seen being held by a homeless demon in the pilot episode of Helluva Boss implies some sort of belief system. Additionally, Vivziepop has mentioned that they also celebrate Christmas but instead of Santa Claus, they eagerly await Krampus's arrival.

Appearance
Demons tend to have an overarching theme to their design. They can resemble animals (some more so than others), objects (such as Vox and some gun-esque demons), homunculi, and more, coming in a wide array of colors, appearances, sizes, and structures based on what creature their character is based on. According to Vivxiepop, Demons that appear more humanoid looking are actually quite rare.

Demons can have either only one eye (Niffty, Cherri Bomb, etc.), two eyes (Charlie, Alastor, Angel Dust, etc.), or even several eyes (Sir Pentious, Helsa, etc.). A feature consistent to all demons is their lack of ears or ear-like structures in their default forms (with the exception of Hellhound Demons and Demons like Husk). Aside from their origins, there are physical differences between the two Demon types; Hellborns don't look human at all and have various different species, such as Imps and Hellhounds. This is unlike the Sinners, whose appearances are highly individualized.

As shown in Helluva Boss, humans are able to see and interact with Demons despite them being classified as souls.

Abilities
Most, if not all, demons share the following traits:
 * Demon Transformation: They have a stronger alternate form as opposed to their default forms which they can transform into when enraged or when in combat.
 * Summoning: Demons have the ability to summon objects and or extra appendages and even assistance one way or another. Examples of this are Angel being able to summon an extra set of arms, Alastor being able to summon an army of shadow creatures, and Charlie being able to summon fireworks out of the palm of her hand.
 * Non-Senescence: Demons do not age, and thus do not die of old age and most of the other natural causes that kill humans. However, they can still be killed once they are hit with any of the Exorcists' weapons or if they have been infected by Hell's own diseases and the like.
 * Unique abilities: Demons have a wide range of abilities that are exclusive to certain demon types or individual demons. These range from Alastor's wide range of powers, Angel's ability to sense storms and jump really high, or Husk's ability to fly.

Reproduction
As mentioned before by Vivziepop, Sinners cannot reproduce, although Hellborns can. In the case of Charlie, her birth is yet to be explained as Lucifer is a Fallen Angel-turned-Demon and Lilith is a human-turned-Succubus Demon.

Sinners

 * Angel Dust
 * Alastor
 * Vaggie
 * Niffty
 * Husk
 * Sir Pentious
 * Cherri Bomb
 * Katie Killjoy
 * Tom Trench
 * Mimzy
 * Baxter
 * Lilith Magne
 * Mrs. Mayberry

Hellborns

 * Charlotte Magne
 * The von Eldritch Family

Goetial Demons

 * Stolas
 * Stella
 * Octavia
 * Stolas's family

Hellhounds

 * Crymini
 * Loona

Imps

 * Blitzo
 * Moxxie
 * Millie
 * Tilla
 * Barbie Wire

Others

 * Lucifer Magne
 * Razzle and Dazzle
 * The Egg Bois
 * Fat Nuggets

Trivia

 * The creature that each Sinner is turned into when they become demons are associated with their human life, or as Viv puts it, individual reflections of their souls. Because of this, their appearances are actually not randomized as many fans have speculated.
 * An example is how there's a reason why Angel's family in Hell all take the form of spiders.
 * When Sinners arrive at Hell, they make up their own name that they are known for as Demons. However, Alastor is excluded since he dislikes giving himself any sort of nickname.
 * Once a demon dies in Hell the dark energy within them wanders off and possesses random everyday inanimate objects such as furniture and tools which explains why some of the things seen in the background and such have functioning eyes.
 * Since Demons can not theoretically die via natural causes, such as illnesses and overdosing, they are able to consume amounts that would be lethal to humans without any risk of dying.
 * This is shown in both Angel Dust and Husk as both are able to consume vast amounts of drugs and alcohol without any threat of death.
 * This does not mean however that there is no negative side effects as Loona makes clear Demons can still get hangovers.
 * According to Vivziepop:
 * Demons have alternate forms if ever they were sent to Heaven.
 * The Sinners have different voices as humans before they were transformed into Demons.