Decim is portrayed as a calm, serious, and highly composed individual, often maintaining an emotionless demeanor while carrying out his duties as an arbiter. He speaks in a formal and detached manner, reflecting his belief that arbiters should not bring personal feelings into their judgments. Decim takes great pride in his role, showing genuine respect toward the souls of the deceased, particularly those who lived meaningful or virtuous lives. When he discovers that he has made a mistaken judgment, he expresses visible shame and regret, highlighting his developing sense of morality.

Although Decim has served as an arbiter for five years, he still struggles to fully understand human emotions, which at times hinders his ability to interpret complex human behavior. Despite this, he gradually shows growth and compassion, beginning to care more deeply about the individuals he judges. Over time, he becomes more empathetic occasionally attempting to lighten the mood with jokes, offering comfort to distressed souls through gestures like hugs, or congratulating them for the lives they led. These actions reflect his evolving perspective and his quiet determination to do what is right, even in a system that discourages emotional involvement.

Decim avoids direct violence whenever possible, preferring to restrain others using invisible threads rather than resort to physical force. He allows guests a certain degree of freedom during their time in Quindecim, only intervening when their actions pose a threat to him, his assistant, or the integrity of the judgment process. His calm authority, growing empathy, and internal conflict between duty and emotion form the core of his character development throughout Death Parade.

Decim is a tall and lean man with a pale complexion and striking bright blue eyes. His most distinguishing feature is his white, tousled hair, which falls in uneven layers, with long bangs that partially obscure the right side of his face, adding to his mysterious and reserved appearance.

As an arbiter, Decim wears a formal uniform that reflects his role and setting. His outfit consists of a white long-sleeved button-up shirt, layered under a black vest, adorned with a red bow tie. He also wears black pants and a black apron, completing the look with polished black leather shoes. His arbiter pin is affixed to the left side of his vest, signifying his official role within Quindecim. The combination of his composed demeanor and formal attire contributes to his professional and somber presence as the bartender and judge in the afterlife.

In the prequel short film Death Billiards, Decim appears as an unnamed bartender and arbiter, observing a judgment between an old man and a young man who are brought to the bar to play a game of billiards. Though his role is not fully explained, this OVA introduces the core mechanics and atmosphere of the later series Death Parade. He is assisted by Chiyuki, who also plays a role in the main story. The OVA serves as a conceptual introduction to the system of post-death judgment later expanded upon in the anime.

In the main series Death Parade, Decim serves as the arbiter of Quindecim, a bar where deceased individuals are sent in pairs to participate in mystery games that draw out the darkness of their souls. In Episode 1, a married couple, Takashi and Machiko, arrive at Quindecim with no memory of their deaths. Following the rules of the judgment system, Decim informs them that they must participate in a game darts, in this case or face an unspecified consequence. Each dartboard section corresponds to a specific body part, and damage dealt in the game manifests as pain in the opponent’s body, raising the emotional stakes of each throw.

As tensions rise, secrets surface particularly Takashi’s suspicion that Machiko may have been unfaithful. In a fit of rage, Takashi pulls darts from the board and attempts to physically attack Machiko, but Decim intervenes, restraining him using invisible threads, thus maintaining order. At the conclusion of the game, Decim makes his judgment: Takashi is sent for reincarnation, while Machiko is cast into the void, based on his interpretation of their actions and emotional responses during the game.

Though Decim fulfills his duties as an arbiter, this early judgment begins to stir doubt in him, as the complexities of human emotion and intention become increasingly difficult to weigh within the rigid structure of the arbiter system. This moment marks a key turning point in his character arc, as he gradually questions the fairness of the judgment system and begins to change through his continued interactions with Chiyuki and the souls who pass through Quindecim.

Threads
Decim possesses the unique ability to generate and control invisible threads, which serve as both a defensive and offensive tool during his role as an arbiter. These threads are primarily used to restrain unruly or aggressive guests, preventing them from causing harm to others or disrupting the judgment process. Despite their delicate appearance, the threads are extremely durable and powerful, capable of stopping individuals mid-motion and slamming them forcefully against walls.

Their versatility is further demonstrated during Decim’s confrontation with Ginti, another arbiter. In their clash, Decim’s threads were strong enough to withstand high-speed water attacks, blocking Ginti’s offensive techniques. He also used them to seize Ginti’s wrist, swinging him with such strength that it sent him crashing through a water tank. In addition to combat, the threads can perform practical tasks, such as lifting and hurling multiple chairs simultaneously, showcasing both Decim’s control and the threads’ raw power.

Though he prefers to avoid direct violence, Decim uses his threads with precision and restraint, intervening only when absolutely necessary to maintain order or protect others, particularly his assistant. His use of this ability highlights his composed nature and his effort to preserve dignity during emotionally intense judgments.

Chiyuki
Chiyuki initially arrives at Quindecim as a guest, but unlike most souls, she retains a significant portion of her memories and is aware that she has died. Unable to persuade her to play a game required for judgment, Decim seeks assistance from Nona, who erases Chiyuki’s memories and extends her stay. Decim then takes her on as his assistant, and over time, the two begin to develop a deep, evolving bond.

With Chiyuki by his side, Decim gradually begins to understand human emotions, often consulting her about her perspectives on the guests and watching how she reacts to the games. He finds her fascinating, appreciating the unique and emotional way she interprets people, which stands in contrast to his logic-based judgments. Their relationship highlights Decim’s growth as an arbiter and his internal struggle with emotion versus objectivity.

Tension arises between them after the judgment of Tatsumi and Shimada, where Chiyuki criticizes Decim for drawing out the darkness in people’s hearts and comparing him to Tatsumi, a serial killer who let others suffer to justify his own sense of justice. This confrontation causes Decim to experience emotional pain for the first time, clutching his chest in confusion and staring at her in stunned silence as she punches him in anger.

Though awkwardness lingers, their relationship begins to heal in Episode 10, when Decim encourages Chiyuki to play a game of Old Maid with a solo guest. As her memories begin to return, Decim expresses his desire to understand her and all human emotions during the time they have left together. By Episode 11, they reconcile fully, and Chiyuki shares her life story with Decim, deepening their mutual understanding and respect.

Nona
Nona is Decim’s superior and oversees his performance as an arbiter. While she often scolds and criticizes him for his occasional misjudgments or emotional confusion, she also sees potential in him. Despite her occasional disappointment, she values Decim’s empathy and perspective, favoring him over the more aggressive and rigid Ginti. Decim takes her feedback seriously and strives to meet her expectations, showing respect and loyalty toward her guidance.

Clavis
Clavis, the elevator operator, does not have many direct interactions with Decim, but the two appear to get along amicably. They exchange polite greetings and maintain a friendly rapport. Clavis shows subtle support for Decim’s unique methods, once telling Ginti to respect Decim’s different approach and let him judge in his own way.

Ginti
Decim and Ginti have a strained relationship, largely due to Ginti’s hostile and competitive attitude. Ginti often acts aggressively and seems to view Decim as a rival, although Decim does not appear to harbor any ill will in return. He remains professional, even addressing Ginti with formal speech. Decim once visited Viginti to ask Ginti’s opinion on the role of arbiters, indicating a willingness to seek different viewpoints, even from those he disagrees with.

However, when Ginti crosses a line by attacking Chiyuki and insulting her for being human, Decim reacts with uncharacteristic anger, using his threads to engage Ginti in a brief but intense battle. This confrontation underscores the depth of Decim’s emotional growth and his protectiveness toward Chiyuki. The fight ends when Nona intervenes, reminding both arbiters of their responsibilities.