The demons* here are in two types:

Example of where the demons were first seen in the story:

In Chapter Five, you see both types of demons. The first one you see is a Born-Demon. That one was hunting for food and Kali was, as mentioned in the story, "easy prey". However, when the Born-Demon saw the Half-Demon, it quickly ran away.  It most likely would have stayed for a fight if it was hungry enough, but although those types pity the Half-Demon, the one standing behind Kali and Gary was--and the Born Demon saw this--specifically looking for Kali.  Staying and fighting would have been hazardous to the Born Demon because the Half-Demons are so much stronger.  Confused?  Read the rest of the descriptions and maybe that will help you.

Half-Demons

These are demons that were once human. However, during the ages 5 through 10, they were kidnapped and during the ages 13 - 18 transformed into demons. They do not share all the characteristics of a regular demons because they are not "complete". Because of this, they are often shunned from the wild demons and disliked, and often pitied. The Full-Demons call them 'incomplete' because they look slightly different.  Half-Demons don't like this, but their loyalty to their master--the one who transformed them into a demon--is so great that they do not fight back unless they are told to do so. Most of them were used for slavery-type work in the Old Times, but now they serve evil Sorceresses and Sorcerers, doing all the dirty work.
     The only Sorceresses in the story that are known are Kali's mother and Gabrielle. I don't mention any other specifics Sorceresses. The reason she is mentioned here is because she is the only one known to Gary and Kali who can actually do the transformation from human to demon. When she does this transformation, the demons obey her.
     Half-Demons are usually taller and heavier than Born-Demons, don't have a tail, and are not at all agile. They live to be about 100 to 200, which is longer than a human's life span, but much shorter than a Born-Demon's life span. They are much less friendly, too.

Born-Demons

These demons are very animal-like. However, they do have feelings, and brains, and know when to leave a Half-Demon alone.  Born Demons are slim and agile, making them dangerous to anyone who isn't fast and doesn't know how to defeat one. These ones have tails, while Half-Demons do not. If you want, you can probably describe them as monkey-like, but they don't have fur all over there bodies.  Instead, they usually just have short, tangle hair on their heads.
     They are most often seen as a purple/violet type color, but do exist in green and orange. Rarely, you find them as a blue color. Their color get darker as they reach old age.  If you see any that are black, then that particular demon is EXTREMELY old (probably around 1,000 years of age) and is very close to the end of life. Demons usually live to be about 500 to 700 years old, and because of this they don't make friends with humans (that, and the fact that they don't like humans since they took away their habitat).
     Given the chance, Born-Demons can be friendly if they trust you and you don't do anything to harm them. They are greedy, though, and let them have their space.  Making them uncomfortable would drive them off and they could turn on you.

Elements (or Types) of Demons

Fire
Earth: Stone and Wood
Water: Water and Ice
Air: Lightning/Thunder and Wind
Special: Dark, Growth, and Time

*Please note that the demons in the story were ones I made up myself. I just used the name demon because it fit them best, and I don't believe that there is any other place where this description of a demon is. 

Note: A picture of a demon will be posted sometime in the near future; I already have one done, I just need to scan it.