Denizen

The Denizens are immensely powerful NPCs, one of which is found in each of the four lands. They are, at first, asleep, but as the player progresses through the Seven Gates, they awaken. Each is said to guard an immense Grist hoard, to be released upon their defeat and used for the Ultimate Alchemy, though Davesprite says that he "had no idea how the denizens worked at all", implying that they serve another purpose. This is probably related to "the choice" each Denizen will offer their opponent and the fact that they are capable of conversing with the players. the Denizens controll the underlings and are responsible to many of the problems found on the players planets.

The Trolls fought denizens of their own, as they are an integral part of the game and its often ignored "plot", but presumably they all (except for Kanaya and potentially Tavros) killed them quickly before the denizens could talk to them. The trolls' and the kids' sessions share at least two Denizens, namely Cetus and Echidna, who were the Denizens of both Heroes of Light and both HeroEs of Space. It is unknown if this holds true for the Heroes of Time or the Heroes of Breath.

Typheus
Also referred to as the "The Slumbering One", Typheus is the Denizen of the Land of Wind and Shade. He is the Denizen of John.

In Greek myth, Typheus was a terrible monster that even the Gods feared, and after he was defeated, was trapped underground. (Specifically, under Mount Etna, the volcano Hephaestus was said to reside in.)

Cetus
The Denizen of the Land of Light and Rain in the kid's session and the Denizen of the Land of Maps and Treasure in the troll's session. As such she is the Denizen of both Heroes of Light, Rose and Vriska.

In Greek myth, a Cetus was a large fish, serpent, whale, or sea monster, which would explain all of the fish being eaten, and why the Trolls' version of Cetus was found amidst shipwrecks.

Hephaestus
The Denizen of the Land of Heat and Clockwork is Hephaestus, lord of the forge. He is the Denizen of Dave.

In Greek myth, Hephaestus lived in a volcano, and was the God of blacksmithing.

Echidna
The Denizen of the Land of Frost and Frogs in the kid's session and the Denizen of the Land of Rays and Frogs in the troll's session. As such she is the Denizen of both Heroes of Space, Jade and Kanaya.

In Greek myth, Echidna was considered the mate to Typheus and the "mother of all monsters" in that she actually gave birth to them.

Trivia
It should be noted that the Denizens of each kid match up with their respective browser of choice. Also they may or may not share appearances with these browsers' icons, but at least their minions should. It should be noted that Cetus' browser icon looks least human and most monstrous of all of the Denizen: Echidna's has a white human torso with breasts and head (perhaps, lacking arms) with two snake tails on its' bottom; Typheus' has a white torso with head and a snake tail on its' bottom; Hephaestus' browser icon version is engulfed in flames, but seems to have a black torso and black head and would be humanoid, making him most human (perhaps, because in Greek myth he was god of blacksmithing and was a human-like god), and Cetus' has a fish body with a white human-like head. This would relate to Rose Lalonde's strong connection with The Noble Circle of Horrorterrors and her least human and most reckless actions in the game (her plan to blow up the Green Sun and her going grimdark), as a foil to Dave's, who was mainly gathering money to buy fraymotifs.

It also should be noted that each browser icon has a coloured part that matches its user's normal clothing: John's Green Ghost Slime and Typheus' snake part are green, Jade's depictions on her shirts are blue, Rose's Horrorterror picture on her shirt is purple, like Cetus' icon, and Dave's record has a red part, like Hephaestus' flames.

Each of the Denizen's lairs are also marked with the same browser icons, at least, Hephaestus' and Typheus' is, thus indicating that the homes of Cetus and Echidna are probably marked the same way.