Ishida Mitsunari is introduced in the Sengoku Basara series as a samurai of immense skill and a retainer completely devoted to the warlord Toyotomi Hideyoshi. In the world of Sengoku Basara: End of Judgement, this loyalty defines every aspect of his existence. He is known by titles such as the Dark King, reflecting both his fearsome reputation and the dark, vengeful path he walks. Before the events of the story, his life had a clear purpose: to serve Hideyoshi and assist in his campaign to unify the land. This purpose is shattered when Hideyoshi is killed, an event that transforms Mitsunari from a disciplined retainer into a being driven almost entirely by rage and a thirst for retribution. In this adaptation, his hatred is specifically directed at Date Masamune, whom he holds responsible for his master's death.

Mitsunari’s personality is intense, rigid, and consumed by his emotions. He is portrayed as a man who is rough and inflexible, completely unable to see past his own need for vengeance. His world narrows to a single point: making those who wronged his lord pay for their betrayal. This obsession makes him appear cruel and relentless, as he cares little for the political ramifications of his actions or for anyone who does not serve his goal. Despite his harsh exterior, he operates on a strict, personal code of honesty. He despises deception and betrayal above all else, perhaps because the initial act of treachery against Hideyoshi scarred him so deeply. He is blunt and straightforward to a fault, never mincing words or hiding his true intentions. His primary motivation is not conquest, glory, or personal power; it is a raw, uncompromising desire to avenge his master's death. Every decision he makes, every army he raises, and every battle he fights is a step toward this singular objective.

Within the story of End of Judgement, Mitsunari serves as a central figure whose quest for revenge sets him on a collision course with other major characters. He commands the Western Army, a coalition of forces that share his grudge against the Eastern Army, but his leadership is defined more by his ferocity in battle than by political savvy. His key relationships are some of the most important aspects of his character. His bond with his strategist, Otani Yoshitsugu, is one of deep trust. Yoshitsugu is one of the few people who can temper, or perhaps even direct, Mitsunari's fury, acting as his anchor to the wider world. His former friendship with Tokugawa Ieyasu, once an ally under Hideyoshi, has curdled into a bitter rivalry, though the depth of his hatred is often reserved for Masamune in this particular storyline. Mitsunari's development, if it can be called that, is a tragic spiral. He is not a character who learns to let go of his anger; instead, he is defined by his inability to do so. His arc explores the hollowness of revenge, suggesting that the very hatred which gives him strength also isolates him and denies him any future beyond the battlefield.

In combat, Ishida Mitsunari is a formidable and terrifying opponent. He wields a long tachi (a type of Japanese greatsword), which he employs in the iaijutsu style, a technique centered on drawing the blade to deliver a single, lightning-fast killing stroke. His fighting style is characterized by overwhelming speed and aggression. He can perform near-instantaneous movements that appear like teleportation, allowing him to close distances in the blink of an eye or strike from unexpected angles. His attacks are often associated with the element of darkness, adding a visual and thematic weight to his brutal techniques. His entire combat philosophy is one of swift and absolute destruction, showing no mercy and leaving no room for negotiation. Abilities like his "flash step" and techniques such as Decapitation and Extermination showcase a lethality designed for a single purpose: to deliver death to his enemies with terrifying efficiency.