'I love the idea that magic and witchcraft and battles between supernatural creatures could be raging all around us but just out of our sight.' 

- Anthony Horowitz 

Underworlders are those with both dark and white sides. Demons are not considered Underworlders because they are all dark.

Those cursed with the inability to see the sun, and the inability to touch holy and sacred things. They draw their energy from the blood of living things. 

distinctive features — pale skin, eyes with startling colours, fangs that can be extracted at will

abilities —

General: Enhanced senses, having speed and agility, and the ability to vanish into the shadows.

Full-blooded: All of the above and Turning humans into one of their kind through taking in their blood and giving them some of their vampire blood. 

age — All vampires are mortal save for full-blooded ones. 

Turned humans and born vampires: Their age is the same as that of their human side. However, since vampires' looks do not age as a normal human's would, they would still look youthful when they're in their fifties or sixties. These vampires have longer lifespans though, and can live to three hundred. 

Full-blooded vampires: Immortal and lives forever unless killed.

additional information — Full-blooded vampires refer to those whose parents are both vampires. They are generally rare, and as a result of their immortality, cannot reproduce. They also cannot come into contact with the sun as they would burst into flames, but full-blooded vampires are stronger than general ones. 

Those with the ability to shapeshift into wolves. They normally form packs so as to protect fellow wolves, as a lone wolf is an easy target. 

The ones whose white side is slightly stronger than their dark side, because they appear to be humans and walk amongst them with no ill effects, except for transforming into a full-grown wolf during nights with full moons against their will. (Of course, mature wolves may control this better) 

Distinctive features — 

Humans: None 

Half-shift: Yellow eyes, claws and fangs may appear 

Wolves: Eyes with startling and unusual colours 

AbIliTies— 

Young wolves: They cannot control their shape-shifting during full moons. 

Mature wolves: They can shape-shift on will, and could turn into a wolf easily without the help of the full moon. 

Half Shift: Typically, mature wolves are those who can use this skill. However, well-trained young wolves may also be able to carry out a Half Shift. A Half Shift allows the wolf to obtain the desirable traits of their wolf form while maintains their human form, such as fangs and claws. 

Wolf form: Enhanced smell, hearing and senses of normal wolves. They may also Turn humans into wolves when they bite them on full moon nights. 

Age — 

Seven human years are equivalent to one wolf year. When a wolf is 2.5~4 years old, they are generally considered mature. 

When a human child is seven, if their parents are wolves, their wolf side would appear.

If a wolf is made, their wolf age would automatically be calculated to their human age. 

Newly turned wolves may be considered young despite their age. That is to say, the skills of a wolf does not necessarily correlate to their age. A wolf may be young, but through training, they can obtain the skills of a mature wolf. It's a matter of hard work, really. 

Also wolves have lifespans of humans, and have the shortest lifespans out of all Underworlders. It is also a price of their white side being stronger than their dark's.

additional information — Ranks of alpha, beta and gamma are given to wolves in a pack. Wolves have their own territory, and does not take kindly to it being trespassed, unless it is a lone wolf seeking for help. 

Wolves have had a tradition of killing in order to take the rank of alpha. However, that tradition had long since stopped. Alphas are now chosen through achievement or democratic opinion, but some barbaric packs may still have the tradition of killing. 

Those with the ability to cast magic are referred to as such, regardless of gender.

Some possess the ability to manipulate elemental forces, some the dead. It depends on the skill the warlock decides to partake, or the skill which is passed on to them. 

Warlocks are generally children of demons and humans. A handful may have met demons, who granted them these abilities and an immortal lifespan. Of course, these abilities does not come without a cost. 

distinctive features — It depends on the demon. Descendants usually inherit the most striking feature of the demon.

For those turned warlocks, the demon usually give them a mark of their choice, as a price for their granted abilities. 

abilities — Again, it depends on the demon. Anything works, given time, patience and training, but most warlocks start with one ability that they inherit or was given, usually the demon's ability. It could be an elemental ability, or anything really. Older warlocks, with increased time and training, may have mastered the skill of using many abilities at once. 

age — Immortal. Warlocks may alter their features with their abilities if they wish, but most warlocks tend to stop growing during their late teens/early twenties, and they remain physically that young forever. 

Offspring of warlocks and humans aren't immortal, however. They will generally age slower than your average human (having twice as long of a lifespan) but they aren't immortal.

additional information — Two warlock parents cannot reproduce. However, if a warlock wishes to sire a child, they may do so with a human. (Not with a demon or another Underworlder; it doesn't work either)

Warlocks may also pass on their abilites to a human child if they so wish, which would strip them of their immortality as well. The human child would then become a warlock themselves. 

The Unseelie Court is composed of 'dark' fairies, and are seen to be more wild and less disciplined. Unlike the Seelie Court, they do not follow any rules save for the orders of their Queen, and even then, generally act on their own whims. 

The fey are beings who possess inhuman features, as well as magical abilities similar to those of warlocks.

Fey of the Unseelie Court are also believed to be offspring of demons, rather than angels, who sired the faeries of the Seelie Court. This rumour has never been clarified, and the Unseelie fey do not like indulging in secrets of their kind. 

Moreover, they are an aloof clan, with the Unseelie Queen and the Seelie King constantly at war with each other. They tend to hate humans, and do not mingle with them unless absolutely necessary, and seem to hate them as well.

Distinctive features — It varies; they usually have 'wilder' features similar to those of demons, such as ram horns. They also do not have wings.

abilites — It varies as well, but generally dark magic. 

Age — The fey are immortal creatures. Offspring of two fey are considered full-blooded fey, and are immortal. However, if the parents are a fey and a human, their offspring will have a longer lifespan. (Like general vampires) 

additional information —The Unseelie fey mostly operates in the darkness. They like to trick humans, and lead them to their demise.

The most peaceful of all Underworlders. They rarely get into disputes with other Underworlders unless they intentionally get into fights with them. 

Even though they are said to be most peaceful, the fey in the Seelie Court tend to be mischevious, and they like to play tricks on humans. Mostly harmless, of course, but sometimes it could get out of hand. 

Despite their mischevious personalities, most of them are willing to help humans if they are in a state of distress, or if the fey wants to help the human.

The only war they are part of is against the Unseelie Court, as their King and the Unseelie Queen has a long-standing rivalry.

The Seelie Court include faeries such as elves and nymphs, leprechauns and other friendly faeries that usually appear in human folklore.

distinctive features — It varies, but the Seelie fey share the same pointed ears. 

abilities — It depends on the kind of fey they are. 

age — Like the Unseelie, the fey are immortal creatures unless the parents are a human and a fey, in which case they would have lifespans thrice as long as that of a normal human's. 

additional information — They tend not to stay in the same place for long, as the Seelie fey likes to travel and wander around. 

The most secretive and perhaps most unfriendly race of all Underworlders. They live in the waters, so only seamen are familiar with them.

The mer-people are a sadistic race; they love to lure humans to their death in dangerous waters, thus the large amounts of humans lost at sea. Some of them are helpful however, though only when you've done them a favour in return. 

distinctive features — A tail instead of legs when in water, gills at the sides of their cheeks and pointed ears. They resemble humans when out of water, except for scales in their skin, which they generally cover with clothes.

abilites — The ability to control the creatures of the sea. Sirens can use their singing to lure seamen to their deaths. They can also change into humans on land, though most wouldn't because they dislike being amongst humans. Other special abilities depend on the type of mer-person they are.

age — Immortal. Offspring of mer-people and humans tend to age slower. One mer-year is equivalent to five human years, but mer-people generally mature at the age of 4-5, and they usually live till the mer-age of 40-50. Similar to other Underworlders, their features do not age as quickly and thus does not reflect their true age. 

additional information — The mer-people generally do not have affairs with humans, but when they do, they can only give birth to one child (with the humans) before turning to foam as punishment. (Males included)