Eiji Kashii, who is known by the zodiac sign of the Ox or Ushii, is a primary participant in the Zodiac War. He is a man in his late twenties to mid-thirties with a lean, muscular build and a height of 181 centimeters. His most distinctive physical trait is a pair of black, onyx-like horns protruding from his temples. He has long, dark brown hair, which is a deep crimson red on the underside, and dark gray eyes with prominent dark circles beneath them, giving him an appearance that is often described as morose or weary. His attire is a striking black matador-style uniform, known as a traje de luces, adorned with intricate gold embroidery.

Eiji is a warrior of exceptional reputation, feared and respected by his peers as the Genius of Slaughter and a natural-born killer. He is said to have engaged in his first battle at the age of five and is known to have never lost a conflict. His personality is stoic, levelheaded, and analytical, approaching situations with a cold and logical outlook. While this demeanor can be perceived as dull or lacking emotion by some, he is also capable of compassion and shows respect toward his opponents. He is driven by a deceptively simple personal philosophy of doing the right thing, which he articulates as deciding to perform a righteous action and then following through with that action. For him, righteous action requires righteous intent, and a failure to act is what allows evil to occur.

Within the story of the tournament, Eiji is the favored warrior to win, even though he possesses no supernatural abilities, relying solely on his immense skill and strategy. His primary role is that of a powerful and methodical hunter who kills systematically. A critical aspect of his motivation is revealed upon the discovery that the warrior of the Rabbit is a necromancer capable of reanimating the corpses of fallen fighters. Horrified by this fate, which he considers worse than death, Eiji resolves to kill all the other participants as swiftly and mercifully as possible to prevent their bodies from being desecrated.

One of the most significant relationships in his past is with Kanae Aira, the warrior of the Tiger. Years before the tournament, Eiji encountered a young Kanae on a battlefield, mistaking her for a lost child, and saved her life. This act left a lasting, if initially unrecognized, impression on her. When they meet again during the Zodiac War, Kanae is a broken alcoholic searching for meaning, and she is drawn to Eiji's unwavering sense of purpose. She confronts him about how he can kill so many without remorse, which leads him to explain his philosophy of righteous action. He eventually returns her feelings, but this realization only comes moments before she is mortally wounded, and he is forced to grant her final request for a quick death.

Throughout the conflict, Eiji experiences a limited but poignant development. His stoic exterior is challenged by his growing connection to Kanae, and the grim reality of the Rabbit's necromancy forces him to confront the possibility of his own death not as a loss, but as a necessary escape from a fate worse than death. In his final moments, cornered by the Rabbit's reanimated army, he decides to take his own life to avoid being used as a puppet, demonstrating his commitment to his code of righteousness even in the face of utter defeat.

As a fighter, Eiji's abilities are entirely martial and grounded in peak human conditioning. He is a combat expert with peerless skill, able to discern and execute strategies with the most efficient use of energy. His knowledge of human anatomy allows him to strike vital points for a quick, clean kill. He wields a mass-produced rapier named Gobouken, which lacks any special properties, making his mastery of the blade even more impressive. His fighting style is defined by precision and economy of motion, a stark contrast to the flashy supernatural powers of many of his opponents.