Disclaimer: DMC and its characters belong to Capcom. Alastor's/Ifrit's personalities are mine.
The Watchers: Whether vigilant or simply curious, they are the Grigori, or "the Watchers." Numbering two hundred, these were the Sons of God, angels, under divine direction to watch over mankind. Within them was knowledge only meant for the celestial. But some of the Sons saw the daughters of men, and began to lust for them. Yet, at the same time they were compassionate in their urge to guide and teach humans - who were meant to learn through life's trials. Whatever their motive, the Watchers took unto themselves mortal wives, and imparted their knowledge to the rest of man (astrology, arts, letters, war, etc.)
God became aware of the Watchers' transgressions, and punished those that had gone against His Word, casting them into a spiritual prison - called the "Abyss" - until the day of judgment.
One Watcher escaped this sentence. Though granted reprieve from a terrible fate, the alternative was not much better, but by his choice, a preferable one. Forever this Grigori would watch over the destiny of mankind, never to directly interfere again. And he must remain useful in his torment: Where ever Hell's taint threatened, he must manifest - no matter how many times over - acting as a sign of impurity to frighten away the innocent, make the hearts of hellkin quake, and to attract the priests and hunters to destroy any wicked spawn. A symbol of defiance and tragedy, forever weighed by time.
The Watcher of Time.
Fallen Angels: Those that chose to mate with human women, the fallen Watchers. Those that chose to side with Lucifer, whose angelic name is no longer, and is better known as Satan. Mundus is a fallen angel, one several generations removed from the power his ancestors once wielded, though no less a formidable being.
There is an unknown number of the Fallen residing in Hell. An estimate is even more difficult to presume because the dark angels - many of them turning on each other years, or even seconds after banishment - are no longer in their original bodies (see Death). The War in Heaven, the cataclysmic battle of the celestials, is said to have cast over a million angels into Fallen exile. Some of their names are: Beliar, Abaddon, Leviathan, Murmur, Anzu, and Astaroth.
Birth: Capable of mating with their own kind, spawn very rarely succeed in producing offspring. Sometimes - either in frustration, the cruel joy of the act, or mere procreation - they venture onto the surface world and mate with humans of the opposite sex, usually amounting to rape with an unwilling partner. Always the chances of yielding mongrel offspring are higher in these cases, something largely dependent on the fertility of the human victim. Sparda was the only devil in Hell's memory to have cared for his soulmate(s). (It's very possible he had other wives, he did live for over a millennia). But even these successes couldn't possibly account for the near countless wicked dwelling in Hell. How is this so? Because Hell itself acts as spawnkind's breeding bank.
As disgusting as it sounds, those hellkin who do not have biological parents, are instead born within an enormously vast cavern of pulsating walls of flesh, the color of the flesh varying from a healthy spectrum, to the infected, bruised, and diseased. Sickening of all are the abscesses dotting the length and width of the chamber walls. Ranging in size of up to five to thirty feet in diameter, each pustule contains an unborn spawn - the literal spawns of Hell - waiting to burst forth and be born.
Because of the almost total lack of natural birth, there are no such notions as "mother" and "father", as well as a slew of other related words, like "family." Instead, there are "clan mothers", "patriarchs", and "Houses". The word "clan" is the closest imitation to "family", but still not viewed as anything worth holding dear. (Exactly how are devils and demons "born" into their respective houses is for another time to tell.)
Death: When angels die, they return to God. When humans die, they are designated a place in the afterlife. When hellspawn/fallen angels die, their darkened souls have one of five paths chosen for them by an unknown force. (Ordered from worst to best)
1) Oblivion: The soul experiences the essence of nothingness. In this space of non-awareness, the soul is forever lost and considered destroyed.
2) Reincarnation: The soul is reincarnated into a newly born body. The body is determined, it is said, by the same force or will that dictates the destiny of every denizen's soul since the creation of Hell (believers of this force have firmly labeled it "The Epoch.") From time to time past-life memories carry over. This usually means the reborn spawn have a clear picture of where it stands in life now, and where they stood in life then. This can either be a blessing or a curse.
Example: A Blade reincarnated into the body of a Beelzebub, definitely ten steps down. A Blade reincarnated into the body of a Frost, a definite improvement. Now take this a step further. Even less frequently occurring than the resurfacing of old memories is the curious ability in which they improve an inferior body, granted if the former body was sufficiently powerful before death. It's almost as if the power of the deceased body - or perhaps the ambition of the soul? - is transferred, greatly enhancing the newborn. (Could Sparda have been one of these? It's a thought.)
3) Sentient Weapon: The soul is given a vessel to inhabit. Traditionally, the vessel is a house's preferred weapon (the equivalent of a coat-of-arms). This is done through precise, often exhaustive ritual exercises - everything from acquiring materials to make the soul weapon, to months straight of self-alienation. Never is a ritual accomplished by force - without the spawn's freewill, the process simply does not work. In the end, the soul is essentially guided into its permanent residence. Sentient weapons cannot be destroyed by any earthly means.
But what's so special about this form of voluntary imprisonment? Fear of Oblivion, fear of a demeaning reincarnation, fear of losing all memories through reincarnation, the power to control the soul's destiny after death, defiance against The Epoch (and sometimes a spawn's reason for wanting to control the fate of a soul). Others, especially ruling patriarchs, do it for prestige, raising the status of their house without ever relinquishing control of the clan. This is very risky, and only done if the patriarch is absolutely certain his position isn't threatened. Sentient weapons are great tools of power, symbolizing the potency of a house.
Anyone with the resources can create a soul weapon.
4) Possession: The soul wanders for a time in a daze after the death of its body. But when a soul wanders for too long, the shock of death wanes, and it begins to act on its own. They feverishly seek the perfect body to take before The Epoch finds them. A spirit will never vie for control against someone obviously stronger than they, instead choosing only challenges they can conquer and - hopefully - that will benefit them most over their previous body.
When possession occurs, the soul of the victim must battle with the invader for supremacy. Temporary insanity ensues. The victim loses, and its soul is violently expelled, destroyed by trauma. The body suffers, as well. The soul is life, with it torn away, the body dies, but remains animate through the invader. Powered by a foreign spirit of death, the new creature born from this is undead, and usually more powerful than either its host or former body.
5) Ascension: The only reason worth dying for. Hundreds upon hundreds of hellkin have gladly thrown away their lives (in some cases repeatedly, given reincarnation and possession) for the vague, intoxicating hope they might ascend. To ascend, is to become godlike, to rival the dark angels themselves. This is the rarest of the rare, a form of life after death so sought after, so hopelessly out of reach, that it is the secret dream of every creature in the Underworld.
Of the handful that transformed, each varied in shape, size, and power level.
The Meaning of Names
Dante - enduring
Vergil - strong
Trish - short for Patricia which means "noble"
Lucia - light
Eva - a form of Eve which means "life"
Alastor - "tormenting spirit" or "Nemesis"
Ifrit - derived from "efreet" or "jin" which were said to be elemental demons of fire