Decim is the central figure of Death Parade, serving as the bartender of the Quindecim bar and an arbiter of the dead. His primary responsibility is to judge human souls who have died at the same time. To carry out this judgment, he hosts pairs of deceased guests at his bar, where they must participate in a game that reveals their deepest emotions and darkest natures under pressure. Based on their actions and the memories that surface during the game, Decim decides which soul is sent to be reincarnated and which is consigned to the void.

In terms of background, Decim is not a human but an artificial being created specifically to serve as an arbiter. One of the other arbiters, Nona, gave him the ability to feel human emotions as part of an experiment, though this gift was something he initially struggled to understand and express. Despite having worked in the Quindecim for around five years, he remained largely detached and lacking in genuine comprehension of the human heart. The mannequins that stand around his bar are not merely decorations; they are the physical bodies of his former guests that he has collected, repaired, and dressed. He keeps them as a way to remember all the souls who came to his bar since his arrival, even though his memories of them eventually fade.

Decim's personality is defined by his profound seriousness, humility, and respect for his role. He is almost always seen with a stoic, unsmiling expression and speaks in a very formal, detached, and disconnected manner. He takes great pride in his function as a judge and firmly believes that every person has the right to be judged fairly. He shows particular respect toward guests who he feels lived their lives to the fullest. While he appears emotionless on the surface, he is not cruel. He dislikes violence and will intervene to physically stop guests from harming each other, using his abilities to restrain them. On rare occasions, when something truly startling or unexpected occurs, he will display subtle signs of surprise. His lack of emotion also leads to moments of unintended humor, as he has been known to crack jokes that fail to land or to act in ways that seem stiff and unnatural to the humans around him.

His primary motivation throughout the narrative is to better understand the humans he is tasked with judging. He is acutely aware of his own shortcomings in this area, and his failure to correctly understand the feelings of a guest named Machiko causes him deep shame, making him feel he had failed as an arbiter. This drives his desire to learn and adapt. His primary method of investigation is observation, as he believes the games he orchestrates will strip away human pretense and reveal a person's true character. By watching how people react when they believe their lives are at stake, he seeks to see the simple, raw emotions of sadness and anger that he believes define humanity.

The most significant relationship in Decim's development is with the black-haired woman who becomes his unnamed assistant, later revealed to be a human soul named Chiyuki. Initially, she serves as a lens for the audience and a voice of reason for Decim, but she quickly becomes far more important. Her presence and her own emotional journey directly challenge his black-and-white view of judgment. Through their interactions, she teaches him about empathy, the weight of memories, and the profound importance of simply living. Their bond culminates in a powerful emotional breakthrough for Decim; as Chiyuki finally comes to a crucial realization about the preciousness of all lives, Decim clutches his chest as if feeling true empathy for the first time, completing the emotional awakening that Nona had set in motion. This relationship fundamentally changes him. Chiyuki is last seen being sent to reincarnation, and the final thing she witnesses is Decim smiling genuinely for the first time.

His other key relationships include his creator and superior, Nona, who watches over his development with interest, and the volatile arbiter Ginti, who represents a more aggressive and contemptuous approach to judgment. Decim generally avoids conflict with Ginti, but he will become irritated and protective of those he cares about if provoked.

Over the course of the story, Decim undergoes a significant transformation. He begins as a near-automaton, perfectly executing his duties without any real emotional involvement. He ends as a being who has learned to empathize. The most visible manifestation of this development is that he abandons his permanent stoic expression and begins to greet his new guests with a warm, genuine smile. Furthermore, the mannequin he creates of Chiyuki is placed next to him at the bar, and it is notably the first mannequin he has ever made that is depicted as smiling.

Regarding notable abilities, Decim possesses the power to create and manipulate thin, yet incredibly strong, threads from his fingers. He can use these threads for a variety of purposes. Defensively, he can restrain unruly guests, stopping full-grown adults in their tracks and swinging them against a wall. Offensively, he has demonstrated the ability to use these threads in combat, such as stopping high-speed projectiles thrown by another arbiter, throwing heavy objects like chairs with great force, and grabbing an opponent to hurl them across a room. He can also use the threads for more mundane tasks, such as hanging decorations, showing the versatile and precise control he has over this unique ability.