NOTE: This verse holds a lot of credit to Joss Whedon's buffyverse. It's not the exact same verse since a lot got changed for the sake of an RP years back and a lot of Buffyverse canon has changed since then (hello wacky comics), but it's similar enough and many of the ideas came from it, so credit is due. If anything, imagine it as an AU-Buffyverse.
So Los Angeles has demons.
Really, are you that surprised?
Actually, Penn's verse is better described as a combination of an infinite amount of dimensions and universes. Some are Earth-like dimensions (like ours and the one Penn resides on), some are Hell dimensions (full of evil demons, salesmen and brimstone), some are heavenly (but can only be accessed after death or if you get a free pass from a deity) and some are just plain bizarre. These can be traveled between either by having the ability to teleport or using portals that can be conjured by certain spells or rituals.
Let's just say right now using portals tend to be a godawful plan. Okay? Okay.
The Earth that Penn resides on is one that contains both humans, demons and other creatures. The existence of demons and other things is kept rather quiet to the majority of the population and the culture is pretty much the same as our modern day Earth.
As for the demons themselves, there are a lot of different species. Some definitely evil, some definitely good and some who have the same moral spectrum as humans. It's complicated.
Important Species to Note
Humans: The dominant species of the planet and not too different from the modern human today. The majority of humans live out their entire lifespan without ever discovering the existence of other species on their planet, much less the existence of other dimensions. Ones who do know the existence of demons will sometimes choose to fight the forces of evil, join the forces of evil or just dabble. Magic is capable to learn for humans, however the level of power varies from person to person - some have enough power to burn the world asunder while others have trouble starting an inch high fire. Occasionally humans are born with innate abilities like being psychic or empathic, but it's rare and usually runs in families.
Vampires: Soulless bloodsucking monsters. These are humans that were turned into vampires by other vampires. They are immortal, super fast, super strong and have heightened senses. They feed on human blood, though it's possible for them to survive on other animal blood too (though it doesn't taste as good). Due to the fact that they are soulless, they are pretty much evil 99% of the time and simply drive to feed on humans. Like humans, they are capable of using magic - but they don't seem to rely on it as much. They are generally the main thing human demon hunters fight and many demons view them as the trash of the demon world.
Demons:
Pixies:
Fairies: FUCKING. EVIL.
Hell Gods: Get in the car.
Powers that Be/Good Gods?: