An amazing place-between-places with absolutely no time relative to that of any other universe. In fact, there really is nothing there, it's just a void abyss with some really well-hidden lighting effects. Anything can happen there... if you can get in.

Gatekeepers
Portal gatekeepers are quasi-real-gaseus-semi-immortal-colored-entities that have the not-quite-unique ability to open portals. Any portals. This includes but is not limited to, Spacial, Chronal, Dimensional, Plainal, and Limbo. They, along with a few other select species, are the only entities that can enter the Portal Realm. The only known place they cannot go is to afterlife realms, except when they've been eliminated from the realms of life.

Purpose: Gatekeepers are created by the Portal Realm to balance out the effects of some certain species of evil portal-hopping demons. They can't kill anything, not even those evil demons, but they can certainly contain them, some of which means are listed below. Otherwise, Gatekeepers just want to have fun and entertain themselves. Most are very easily amused.

Shape-shifting: Gatekeepers can turn into anything they want to be when in the Portal Realm. When they enter the Mortal Realm, they change form to become whatever the portal wishes them to become, which can be quite fascinating at times. Some have found ways to control the Portal's randomness, but this is often null by virtue of the fact that Gatekeepers simply ENJOY randomness.
Normally the portal tries to help the Gatekeeper blend in with the dominant species of that universe, but it most often fails... miserably. Eg; flaming hair, mixed up clothing styles, being gelatin.
Not that a Gatekeeper would care, as their only real sense is that of humor. They have no fear, no moral restraints. That would be deadly combined with their semi-immortality, but since the thought of bringing death upon anything is locked from their souls and they wouldn't normally think to harm anything other than those demons they exist to balance out, there's no problem of Gatekeepers bent on multi-verse domination.

As for being semi-immortal: That's because they have only one weakness; each color gatekeeper can potentially be killed by one or more of the distinct species of demon (see link at bottom).
That, and should the thought of bringing death upon something ever cross a Gatekeeper's mind, they would spontaneously implode, for good. Their spirits would be sent into the relatively inescapable Death Realms.
While in Mortal Realms, they can never take their true gaseous form. The mortal bodies they take on upon entering a Mortal Realm do age, as well. The bodies themselves are not immortal, just the spirit of the Portal Gatekeeper.
So, should you blow off the head of a Portal Gatekeeper, a portal would open within their body, and they would implode, the portal taking them back into the Portal Realm. There, they may decide to re-enter your world in a hover over your head, stick a pound cake down your shirt back, then celebrate by teleporting every one to a raft stuck up on a glacier in Antarctica and throw a birthday party for every snowflake in sight.
Deagan has been known to do that.

Known Gatekeepers
Paws: she's coordinator of this site; it was all her idea, blame her. "I heard that!"
Garrett: she co-founded the original site back in January of 1998.
Ezra: a cohort in the insanity, she has helped much with Demon developement."Cookies?"
Bunnie: she's come up with an overload of fanfic ideas and characters.
Yoma: he's the Inverse & Kuboshi GM and drain demon bait. "Space sharks!"
Ryoga: he's a new yet nicely random Gatekeeper. "Fishcakes!"

NPC Gatekeepers
Deagan: he's the oldest known Gatekeeper.
Stormy: he's the current youngest, and is considered Paws'.

Gatekeeper/Demon Relationships:
These are just the known weaknesses and resistances;