Dii serves as the primary antagonist of Utawarerumono, a figure whose true nature is intertwined with the very fabric of the world's creation. His physical form initially belonged to a noble scholar of the Onkamiyamukai, a person who was also the mentor to the character Ulthury. This original individual met his end when he wandered into a cave where the soul of the god Uitsualnemetia was sealed. Upon witnessing the sealed god, the being's darker half was released and possessed the scholar's body, completely erasing his original personality and giving rise to the entity known as Dii.

In terms of personality, Dii is cold, calculating, and thoroughly ruthless. He is a master manipulator, using other people willingly as tools to achieve his grand designs. Despite his ruthlessness, he operates under a specific set of rules and will honorably fulfill his end of a bargain. For example, he spared the life of Kuuya following the sacrifice of Genjimaru, even though his pact with her predecessor would have normally required her eternal servitude. However, any sense of honor does not prevent him from mercilessly killing those who turn against him, as he did with Genjimaru himself, whom he regarded as his only friend. He acknowledges that due to their opposite natures, he and Hakuowlo were never meant to meet and fight, suggesting an understanding of the cosmic balance that binds him.

Dii's core motivation is to create social progress through conflict. He believes that by instigating wars, weak races will be eliminated, allowing the strong to survive and lead the people to greater heights through a process of natural selection. All the races of the world are considered Uitsualnemetia's children, and Dii seeks to catalyze their evolution through this brutal philosophy. To achieve this, he aligns himself with several nations, most notably Shikeripechim and Kunnekamun. He revealed Hakuowlo's true identity to Niwe, prompting Shikeripechim to declare war, and provided the Abh-Kamu, a powerful weapon, to Kunnekamun so they could rise against their oppressors and become his people. He also used sorcery to deceive Orikakan into declaring war on Tuskuru.

Before his possession, Dii was a scholar and mentor to Ulthury, a connection that hints at the tragedy of his transformation. His most significant relationship is with Genjimaru, whom he considered his sole friend. Even so, when Genjimaru turned against him, Dii killed him without emotion, a stark display of his inhuman nature. His entire existence is also defined in opposition to Hakuowlo, who is the host of Uitsualnemetia's good half. They are two halves of a single divine entity, and their final confrontation is inevitable. At the end of the story, after losing the fight, Dii, bound by the rules, intended to go into a long sleep. However, Hakuowlo chose to fight him, and the two engaged in their god forms. Dii nearly won, but Hakuowlo ultimately decided to fuse the two halves of Uitsualnemetia together before sealing himself away, thus destroying Dii's mortal form and reuniting the dark half of the god's soul with his own.

Dii possesses notable abilities that set him apart from Hakuowlo. Unlike Hakuowlo, who struggles with his divine powers, Dii has complete conscious control over his abilities. He is able to dispose of powerful warriors like Genjimaru without visible effort, demonstrating a level of mastery and strength that makes him an incredibly formidable opponent. In the related title Lost Flag, Dii appears in a different light, as a friend and ally to a character named Akuta. In this depiction, he shows signs of already being under Uitsualnemetia's influence, hinting at the tragic path that will eventually lead to his full transformation into the dark god's host.