The Devils are peculiar infernals, and serve a role akin to gods among their lesser kin. It is not recommended to refer to the Devils as Demons to their face, as those times such have been done, it was seen as a insult by the demon in question.
There are five devils in total, and their degree of power is, quite frankly, on par with most divines; even the more powerful ones. But, even as they possess great personal power, most of the Devils seemingly avoid the center stage. If this is because they don't want the other devils and the gods to combine forces against them, or because they find it more interesting that way, is unknown.
While the Demons are chaotic in nature, the Devils aren't quite as united in disposition. Each of the five has its' own motivation and interest. They are, on the other hand, occasionally overlapping; and when two Devils have clashing interest, there's often much destruction to come.
Belial: Change, knowledge, curiosity; these are three traits that well describe the motivations of Belial. He is a scholar and manipulator by nature, and greatly enjoys meddling with the world of mortals. Though he is often seen as one of the "lesser evils" among the Devils, as he has no interest that would relate to destruction or death. He is instead more curious to see what would happen next, and he doesn't often intervene personally, unless his favoured "actors" would be threatened with destruction. Belial often appears in the shape of the classical "human devil" as portrayed in media. He is shown as a tall, lithe man with oiled black hair, and a well-kept van dyke beard, and he is nearly always either interestingly watching or somehow responsible whenever something important happens.
Belial doesn't often take part in deals, as there isn't often something that mortals can give him that he actually wants; as his desires aren't physical. There are, on the other hand, cases where he was offered something very interesting, which caused him to grant boons to the provider. It is worth mentioning that if you make a deal with Belial, and you do not fulfill your end of the bargain, the results will be... destructive.
Belial often hands out sources to people that he find interesting. Those who are granted such sources are people of special interest, and Belial often intervenes (mostly behind the curtains) on their behalf should he feel his chosen "actors" need it. A person chosen by Belial is always destined for greatness. If this is for the better or worse, only they themselves can decide.
Azazel: War, Combat, Personal strength; These are three traits that well describe the motivations of Azazel. He is a warrior and a force of nature that is nigh impossible to stop once he starts moving. Azazel's true goal in existence is to find worthy challenges, and fight them in a manly fashion. He normally ends up rampaging through worlds' heroes and pantheons before moving on, like a hurricane sweeping through the land. It is worth being said that Azazel is by no means a mindless berserker, and adheres to a code of honor that he never breaks. He is also considered the least dangerous of the Devils, as you have to either challenge him, or be akin to a god of war, to risk death at his hands. Azazel also respects courage, valor and martial honor, and is very generous to those he considers "cool people", and is one of the most all-around bro supernatural entities; except possibly the god of keggers. It is also said that the alcohol that is brewed by Azazel is some of the best to have ever existed in the multiverse. He is also said to be fairly close to Belial, and when either of these have an issue they themselves cannot handle, they usually go to the other first and foremost.
What Azazel would demand of those seeking to make deals with him is simple: Find the biggest, baddest thing and punch them in the face. If it then defeats the champion, bring it to Azazel to fight. He has no real desire for wealth, worshipers or souls, and would rather that everyone first have a brawl, and then get drunk together. The order of these two may vary.
To be gifted a source by Azazel, you have to be either really badass, or a total bro.
Lilith: Corruption, Depravity, Excess; These are three traits that well describe the motivations of Lilith. She is the only Devil that identifies as Female, but this description may be... misleading. Being ultimately above mortal definitions such as gender, Devils may take on the shapes of anything they desire; something that Lilith takes great pleasure in abusing. She is able to see the desires of every mortal, no matter how well he or she is at hiding it from both others and themselves; and Lilith uses this knowledge to break and toy with mortals. She finds great pleasures in peeling away all reservations and turning the mortal to corruption and depravity. When interacting with mortals, she takes on the shape of their greatest desire; should that desire be a person (or type of person); and when interacting with mortals in a group or social setting, or simply mortals with no particular sexual or amorous desires, she takes the shape of a beautiful, pale, black-haired woman. She is known for often spending her time among the wealthy and influential, and revels in excessive parties and lavish situations.
Lilith is one of the more "orthodox" devils, and many of her deals are for the services of mortals in life, or the souls of themselves or others to serve her forever in afterlife. In return for these things, Lilith gladly rewards her slaves with worldly pleasures and power, granting them their greatest desires.
For people that have been of particular use for Lilith, she often offers them gifts and boons to repay these services; She is also a spiteful devil, and if the powers are used in ways not desired by the devil, she is quick to revoke them. If this happens, the former favoured person will know why "hell hath no fury like a woman scorned" is doubly valid when talking about a devil.
Lucifer: Domination and Conquest; These two traits well describe the motivations of Lucifer. He is the dawn star, the first and most powerful of the Devils. He is one half of what people think about when they talk about "the devil". He is at war with the multiverse, and he will not rest until all bow before him, and him alone. This doesn't make him particularly popular among those who would oppose him; that being gods, angels, demons, or other Devils. Lucifer commands hosts of Demons and Fallen angels on a grand crusade everlasting. To him, you either bow... or you break. He does not care much which. He is considered among the most dangerous of the devils, as you, no matter how small and insignificant, cannot avoid his vision of conquest.
He will reign, or no one will.
To make a deal with Lucifer is to make yourself a tool, a weapon of his crusade. Those who make deals with him are often tasked with expediting his ambition to conquer the world, and allowing his hosts access into the world.
He grants sources primarily to those who prove themselves to be good little servants of his cause. And he is generous to those who prove their loyalty. He is wrathful against those who would betray his legions, and deserters are dealt with harshly.
Mephisto: Death, pain, fear; These three traits best describe the Devil known as Mephisto. He is considered the most dangerous of the devils, since his desires don't have anything to do with conquest, control or manipulation. No, Mephisto's only desire is to spread fear and suffering wherever he turns his gaze. Those foolish enough to evoke his presence quickly regretful of their mistake, as the lord of torment takes them on as his latest toys, to be added to his galleries of despair. He is the second half of what people think of when talking about "the devil", and his domains can truly be called hell.
If you summon his presence, and avoid a fate worse than most could conjure up in their worst nightmares, you are either insane or sick beyond measure to make a deal with this devil. His slaves; for there is no better way to describe those holding contracts with this devil; teether of the rim of the abyss. They are granted great power, and are sent out to wreak havoc and suffering upon the mortal realm. Serial killers, cannibals, psychopaths... these kinds of people are drawn to the service of the lord of torment.
The lord of torment isn't one to hand out his gifts generously, but those who are granted his boons are often the worst imaginable of humanity's filth. This is how super villains are born.