The Demon Lord, alternatively translated Monster Lord, Mamono Lord, Overlord or sometimes Maou (Kenkou Cross considers the "demon lord" translation misleading, as the position is not meant to imply that the ruler is necessarily a demon type monster or only rules over demons), is the leader of all the monsters in the Monster Girl Encyclopedia world. There have been multiple Demon Lords, and the title may have been held by very many monsters since the creation of the world. Previous holders have been bloodthirsty anthrocidal killers in the classical "monster" tradition, but the current incumbent, a succubus, is rather more a lover than a fighter.
The Current Demon Lord
The current Demon Lord is responsible for transforming monsters into the human-loving creatures we know of today, and is the mother of all the lilim. Other details on her personality and interests (beyond sex) are sketchy, in spite of her central role in the Monster Girl Encyclopedia. She is at least mentioned to deeply love her husband, and the human species in general. Appearance-wise, "[lilims] have pale skin and pure white hair, which they share in common with the Demon Lord and red eyes full of demonic energy". Also, the story on the punishment of the Tentacle Forest probably relates to an adventure she personally embarked on early in her reign (although the "white-haired succubus" to which it refers could conceivably be one of her daughters instead, its unlikely given how it seems to have happened not long after monsters became monster girls, and thus there was probably not enough time for any lilim to have been born and matured yet).
Although all monsters would agree that the Demon Lord is, at least in a nebulous sense, their "leader", the degree of control she exerts over different factions and species varies considerably. For example, while the monsters of Zipangu appreciate and respect her for giving them female forms, they are in practice almost completely autonomous. Nevertheless, the Demon Lord's metaphysical sovereignty over the monsters does affect their behavior: just as her succubus nature influences their bodies, so too her personality influences their minds, giving all monsters a preference for monogamous relationships and not stealing other girls' husbands (though their own cultural and species norms have some influence too).
In the previous era, usually, Demon Lords came to power over many years by butchering their way to the top of the pile of slavering inhuman monstrosities, with the tacit approval of the Chief God, with their function being to organize the monsters and lead them on campaigns of such massive scale that humanity as a whole is culled and falls in numbers substantially. For reasons that are not completely known (there were many factors at play), the present Demon Lord was able to short-circuit this process, gaining the title by acclamation from the other monsters (though she did fight for it as well). While most monsters in former ages literally ate humans for breakfast due to instincts instilled by the gods, the Demon Lord was instead motivated by a deep love of those creatures that were supposed to be her enemy (she was an example of the rare monsters that were able to resist the human killing instinct, and even come to love humans), and so she set in motion a plan to end their eons-long conflict forever, by transforming monsters into monster girls and trying to fuse the two species into what would in effect be one united "human" species (incubi as the males, and monster girls as the females). Together with her human husband (the chosen hero of the time and now even more powerful as an incubus), she drove off the Chief God (who had come down to kill both of them) in battle and began to focus on expanding her influence on the world.
The Demon Lord's original plan - to cast a grand sorcery that enabled humans and monsters to interbreed - was only partially successful. Although it created the monster girls and removed their human-killing impulses, at present their coupling with humans can only produce female offspring: that is, more monster girls, whereas the plan was to have monster girl daughters and incubus sons. and thus produce a self-sustaining united species. Obviously, the current situation represents potentially a very slow-moving (its been a thousand years and there are still plenty of humans) demographic train crash that'll eventually lead to the extinction of monsters and humans. In order to build up magical power to push the final stage of her plan through the Chief God's metaphysical resistance, the Demon Lord spends most of her time holed up in her castle in Royal Makai screwing her husband.
However, the Wandering Scholar advises that if one wishes to, they can seek her and her husband out and consult them for their knowledge (assuming they can make it to their castle without ending up captured by a monster). The scholar himself is stated to have interviewed her in person (perhaps thanks to the protection from other monsters offered by his Leanan Sidhe traveling companion).
World Guide 2, in explaining magic gems, remarks that her (and her husband's) power is so intense that no magic gem has yet been crafted that is capable of holding it.
Succubus Notebook gives the info that she is believed to be the first succubus Demon Lord in history (succubi were at the time usually mid-rank monsters and thus unable to win the competition to gain the title of Demon Lord), and that even before ascending the throne and gaining the power associated with her title, she was the most powerful of all succubi living at the time.
World Guide 3 expands on our knowledge of her a bit. Namely we learn that it is rumored she has adopted at least one human girl, and used her incredible powers to make her into a succubus that is a lilim in every sense aside from her parentage. We also get a pretty clear indication (from the name of a legendary item crafted by accident by her) that the Demon Lord's name is Lilith. Said item also gives us our first clear indication of her looks beyond her similarity to her daughters, the item depicting a very elegant and voluptuous succubus with very long hair, rather intricate horns (compared to other succubi) and magnificent wings.