Kyouji Mujo serves as the primary antagonist of the series, a figure whose overwhelming ambition and thirst for power drive the central conflict of the later narrative. He is an Alter user originally from the Mainland, a region distinct from the Lost Ground where much of the story takes place. His past is largely shrouded in mystery, but it is known that he grew up in harsh wasteland conditions. His deep-seated disgust at living in the shadow of more powerful and prosperous Alter users, including those within the organization known as HOLY, ultimately led him to willingly undergo the process of refinement to enhance his own abilities.

Kyouji Mujo is defined by an unquenchable greed and a pathological desire for absolute supremacy. He is fundamentally a madman and a megalomaniac, driven by an endless hunger to possess all power for himself. He speaks frequently of attaining the Other Side, a parallel dimension that is the source of all Alter power, which he believes will grant him ultimate strength. His personality is characterized by a complete disregard for the lives of others, as he is willing to sacrifice anyone, ally or enemy alike, to achieve his goals. He possesses a smug and sadistic demeanor, often gleefully tormenting his opponents, but this confidence can quickly give way to sheer terror and panic when his plans are thwarted by an enemy he cannot overpower.

Mujo’s primary role in the story is as the hidden force behind the sinister activities of HOLY, the organization tasked with controlling Alter users. He orchestrates events from the shadows before seizing control of HOLY for himself, discarding its previous leadership. His master plan hinges on manipulating the two most powerful Alter users, Kazuma and Ryuho, into battling each other. He believes that a clash between these chosen ones will tear open a rift between worlds, creating a door to the Other Side that he can then exploit. After finally making contact with the Other Side, he absorbs the immense power of a Crystalline Alter Embodiment, a monstrous entity responsible for the death of Ryuho's mother, which transforms him into a nearly unstoppable force. In his mutated, final form, he becomes a grotesque monster that represents an accumulation of negativity, and he goes on a rampage, destroying his own headquarters in a berserk attempt to annihilate Kazuma and Ryuho.

His key relationships are defined by his rivalry with and desire to consume the power of the two main protagonists. He sees both Kazuma and Ryuho as means to an end, tools to open the door to the Other Side whose powers he ultimately intends to absorb. He respects no one and forms no genuine bonds, viewing all other characters as either obstacles or stepping stones to his own apotheosis. His relationship with the Crystal Alter is purely parasitic, as he drains its energy and then wields it as a weapon, summoning it to fight on his behalf or merging its limbs with his own body to create his signature attacks.

The character’s development follows a trajectory of escalating power and monstrous transformation, rather than moral growth. As he acquires more abilities through absorption, his humanity visibly erodes, culminating in his final mutation into a towering, multi-limbed creature. However, this ultimate form proves to be a Clipped-Wing Angel; his newfound power is fleeting. When faced with Kazuma and Ryuho, who have achieved their own ultimate Alter forms rooted in pride and purpose rather than simple greed, Mujo is utterly outmatched. He is horrified to discover that his most devastating attacks cannot even scratch his enemies. In a moment of panic, he is destroyed by Kazuma’s final attack, his body disintegrating moments after he believed he had finally satisfied his life’s ambition.

Kyouji Mujo’s notable abilities revolve around his unique Alter power, known simply as Absorption. This power allows him to drain the Alter energy from other living beings through his breath or hands, integrating their unique energy signatures into his own body. This process not only grants him the ability to replicate the abilities of those he absorbs but also disintegrates the victim's physical form. His most significant acquisition is the power of the Crystal Alter, from which he steals the formidable energy-based arms he calls White Trick and Black Joker. These limbs can function as high-velocity rotating drills that tear through tough materials and can unleash powerful electric shocks. He also gains the ability to summon the Crystal Alter itself as a standalone entity to fight by his side. Other stolen abilities include generating an electric field to paralyze opponents and technopathy, which allows him to restore power to massive structures. After his mutation, his powers shift away from absorption and toward raw, destructive output, including omnidirectional energy tendrils capable of leveling everything for kilometers and a massive energy laser, though his enhanced body in this state is vulnerable to focused attacks.