Clevatess, also known as the Dark King of the South, is one of the Four Beast Kings, immensely powerful creatures who act as living barriers confining humanity to a portion of their world. Known specifically as Clevatess of the Moonlight, this being rules over the southern lands from a territory called the Luna Mountain. For a thousand years before the story begins, Clevatess lived as an isolationist, rarely interacting with humans and never deliberately attacking their settlements, considering them beneath notice. This long period of quiet ended when thirteen heroes trespassed into its domain in an attempt to slay the Beast King. The attack proved completely ineffective, but it successfully provoked Clevatess into a devastating retaliation. Leaving its territory for the first time in centuries, Clevatess launched a direct assault on the capital of the kingdom of Haiden, razing the royal palace, slaughtering the king, and annihilating the kingdom's leadership in a brutal display of power.

In the midst of the destruction, Clevatess was stopped not by force but by curiosity. A dying boy buried in the rubble, a young servant named Shiro who had been protecting the infant Prince Luna, begged the Beast King to spare and raise the child as a way to prove humanity's worth. Intrigued by the proposition and perhaps experiencing a rare whim, Clevatess agreed. The Beast King took the baby, who would become known as Luna, and thus began an unexpected journey. Clevatess's initial personality is marked by a profound lack of understanding of human life. As an ancient, immortal being, its perspective is clinical, detached, and often cruel in its efficiency. It is not motivated by malice in the traditional sense but by a cold, analytical curiosity. Clevatess views human customs, morality, and suffering with an anthropological gaze, cataloging hypocrisies and brutalities without sentiment. This ignorance leads to humorous yet telling moments, such as when it resurrects the fallen hero Alicia, the only woman among the thirteen, and immediately commands her to breastfeed Luna, having no concept that women cannot produce milk on command. Similarly, its initial attempts at childcare are wildly inappropriate, including dumping the infant into a pit of dung beetles to serve as a diaper-changing solution, believing the baby would grow to adulthood in a matter of nights.

Despite its fearsome power and monstrous nature, Clevatess possesses a strong, if alien, sense of honor. As a king, it holds itself to a high standard and will not break a promise once made, even for pragmatic reasons. This code of conduct forms the basis of its relationship with Alicia, whom it resurrects as a form of undead slave bound to its will, solely to have a caretaker for Luna. Clevatess's primary motivation throughout the narrative is, therefore, learning. It seeks to understand humanity, to decide whether this flawed and violent species deserves to continue existing or if it should be exterminated. The baby Luna is the catalyst for this thought experiment, and the resurrected Alicia becomes its unwilling window into the human condition. To navigate the human world without causing mass panic, Clevatess adopts a human disguise, taking the form of a dark-haired young boy named Clen or Klen. This form is based on the boy who begged for Luna's life, and crucially, it renders Clevatess far more vulnerable, its immense power seemingly diminished or suppressed while in this shape.

Over the course of the story, Clevatess undergoes a significant if subtle development. Its initial contempt for humanity is gradually eroded by direct experience. The growing attachment to Luna, whom it begins to protect with a fierce, paternal instinct, becomes a central part of its character. It chastises Alicia for any perceived failure in caring for the child and develops a grudging tolerance for the hero's stubborn morality. Key relationships are thus central to its arc. The bond with Alicia is jagged and filled with tension; she is a slave who refuses to surrender her sense of right and wrong, and her challenges force Clevatess to confront its own assumptions. The bond with Luna is more direct, a wordless connection that transforms the Beast King from a detached observer into a reluctant protector. A pivotal moment in its development occurs when a deeper mystery about the world's controlling forces leads Clevatess into a crisis of purpose, questioning whether it or any being has true free will. It is Alicia's forceful, telepathic question about the color of Luna's eyes that snaps it from its despair, forcing it to recognize the tangible, living reality of its ward and its companions over the abstract dictates of ancient powers. This realization solidifies its commitment to the small, unconventional family it has formed.

Clevatess's notable abilities are as formidable as its title suggests. In its true form, it resembles a house-sized, pitch-black wolf-like creature with a long horn on its forehead, several sharp, prehensile tails, and the ability to manipulate shadows. It can cast a shadow that consumes people, and its tails can teleport through the shadows of its enemies to impale them, a devastating and nearly unavoidable attack. As one of the Four Beast Kings, its power far exceeds that of any ordinary dark beast or human. Clevatess is also a skilled analyst, able to grasp the mechanics of magical weapons and abilities like Regalia after witnessing them only once. Furthermore, as a Devil Beast King, it possesses a form of necromancy, capable of raising the recently deceased by imbuing their corpses with its own blood, which is how it resurrected Alicia. While in its human guise of Clen, Clevatess's power is greatly reduced, though its true form remains hidden within its own shadow, and it can use its blood to manifest a small, super-deformed version of itself for communication. This combination of overwhelming might, alien intellect, and a slowly awakening sense of care makes Clevatess a complex figure who blurs the line between destroyer and possible protector.