Enma Jr, most commonly referred to as Koenma, is the son of the supreme ruler of the Spirit World, King Enma, and serves as the prince and eventual ruler of the afterlife realm. Despite his youthful and often childlike appearance as a toddler, Koenma is actually over seven hundred years old, a fact that contrasts sharply with his small stature and the pacifier he perpetually keeps in his mouth. This pacifier is not merely an affectation; it serves as a powerful repository for his spirit energy, allowing him to unleash the Mafuukan, the Spirit World's most potent defensive spell, which can seal demons of significant strength.

In his role as the administrator of the Spirit World in his father's frequent absence, Koenma is the official who judges the souls of the deceased, deciding their fate in heaven or hell. He is also the singular individual responsible for appointing and overseeing the Spirit Detective, the human agent tasked with protecting the mortal world from supernatural threats. Initially, Koenma’s personality appears spoiled, bossy, and somewhat cowardly, often shouting orders at his ogre attendants and relying on others, like the ferry-girl Botan, to carry out his will. He can be evasive and prone to blaming others for his own mistakes, which often undermines his attempts to project authority.

However, this superficial demeanor masks a deeper sense of responsibility, fairness, and genuine compassion. Despite his occasional laziness, Koenma cares deeply about his duty and the well-being of his employees. When Yusuke Urameshi or his allies face true danger, Koenma abandons his cowardice and acts decisively to help them. His commitment to justice is absolute, even when it places him in direct opposition to his own father. Upon discovering that King Enma had deliberately allowed demons to invade the human world to stoke conflict for his own purposes, Koenma chose to betray his father, overthrow him, and take his place as the rightful ruler of the Spirit World.

Koenma’s most significant relationships are defined by his role as a leader and authority figure. He shares a close working bond with Botan, who serves as his cheerful and capable messenger. He acts as the demanding but well-meaning superior to the current Spirit Detective, Yusuke Urameshi. Their relationship is often combative, with Koenma frustrated by Yusuke's rebellious nature, yet he deeply respects the young man's strength and integrity. The central conflict in his personal development is his relationship with his father, King Enma, a powerful and short-tempered being whom Koenma initially fears above all else. His journey from a frightened prince to a courageous leader is solidified when he stands up to his father's corruption. He also carries a heavy sense of responsibility for the previous Spirit Detective, Shinobu Sensui, whose descent into villainy Koenma feels he failed to prevent. He is willing to risk eternal imprisonment to atone for this failure by using his Mafuukkan to seal Sensui away, sacrificing himself in the process.

In terms of abilities, Koenma’s power is closely tied to his pacifier, which stores the spirit energy needed for the Mafuukkan. While his true form is that of a small child, he has the ability to transform into a mature, tall, and handsome teenage or young adult form, which he uses to command more respect. In this adult form, he loses the pacifier and appears as a composed leader. His knowledge of the universe is extensive, and he is far more powerful than his usual antics suggest, capable of sealing even S-class apparitions with ease in later events. Through his trials and difficult choices, Koenma evolves from a comical, bumbling manager into a just, courageous, and effective leader of the Spirit World.