Katakura Kojuurou is the most trusted retainer of Date Masamune, the "One-Eyed Dragon" of Oushu in the Sengoku Basara series. His character is deeply defined by his role as Masamune's right-hand man, a position he has declared for himself and embodies completely. While the series never gives a specific age, he is portrayed as being in his early to mid-thirties, roughly a decade older than his lord, and his persona is modeled in part on a archetypal yakuza gangster, evident in his direct manner of speech and his fierce, unwavering loyalty.

Kojuurou's background is not extensively detailed in the anime, but key flashbacks reveal a significant transformation. In his youth, he was hot-headed, reckless, and quick to anger. When Date clan elders considered disinheriting the newly one-eyed Masamune, a young Kojuurou physically assaulted them and famously declared himself to be Masamune's right eye. It is strongly implied that from that moment on, he chose to deliberately restrain his own wild nature to create a space for the young Masamune to grow and lead as the clan's recklessly ambitious figurehead. This makes his outward persona a carefully maintained construct, a necessary calm to balance his lord's storm.

His personality in the present day of the series is defined by a stoic and collected demeanor. Often compared to ice, he is gruff, speaks with direct and sometimes rude bluntness, and rarely displays overt emotion. This calm exterior is not an absence of feeling but a product of immense self-discipline. His primary motivation is the success and well-being of Date Masamune. He is not a blind follower; rather, he serves as a crucial advisor and moral compass, willing to defy or even physically restrain his lord to prevent him from walking a path to ruin. In one notable instance, he knocks Masamune unconscious to stop an injured man from rushing into battle, an act another warlord points to as a rare combination of skill and absolute trust. Beneath this controlled surface lies a deeply caring individual. He risks his life for ordinary Date retainers, allows himself to be captured to spare peasants from being slaughtered, and demonstrates a strong sense of justice, caring for the needs of non-combatants even in a world defined by martial rule.

Kojuurou's role in the story is inextricably linked to Masamune. He is the strategist, the guardian, and the steady hand that guides the Date clan. His strategic mind is exceptionally sharp, capable of dissecting and countering the plans of other master tacticians like Takenaka Hanbei in moments. While he facilitates much of Masamune's success, his personal journey often involves protecting his lord from threats both external and, at times, from his own recklessness. His relationships with others are largely defined by this loyalty. He shows a fierce, almost paternal protectiveness towards Masamune, and when his lord is attacked through underhanded means, Kojuurou will personally hunt down the offender, dropping his stoic facade to unleash his full, unrestrained fury. His key antagonistic relationship is with Takenaka Hanbei, who recognizes Kojuurou's skill and attempts to forcibly recruit him, seeing him as a worthy intellectual rival and a possible successor.

The most significant development for Kojuurou is the occasional emergence of his hidden, more primal self. The controlled, icy retainer suppresses a former self that was hot-headed and wild. When his anger is truly ignited, particularly by threats to Masamune, he enters a state known as Mugetsu Gokusatsu, or "Slaughter Mode". In this state, his fighting style becomes ferocious and unrestrained, like that of a berserker, and he stops holding back his true power. His mentor-like relationship with Masamune also reveals moments of quiet trust and even gentle smiles, suggesting a deep bond forged over many years, where he can drop his guard when they are alone. The oath inscribed on his sword is described as a seal on his inner darkness, and his character arc involves the tension between maintaining this seal and the necessity of unleashing it to protect all he has sworn to serve.

In terms of notable abilities, Kojuurou is an exceptionally skilled and powerful warrior whose straightforward style belies his immense strength. Unlike many characters who wield oversized or multiple weapons, he fights with a traditional daisho, a pair of katana and wakizashi. His sword style is strength-based, practical, and devastatingly effective, with each blow possessing enough power to send multiple armored soldiers flying. There are numerous hints throughout the series that he may be an even more skilled swordsman than Masamune himself. His elemental affinity is lightning, which he typically does not use. It manifests as a crackling blue aura that adds an extra "kick" to his attacks, but it only surfaces when he is truly enraged and has abandoned his restraint. His most prominent technique is the "Mugetsu Gokusatsu," an activation of his Slaughter Mode which dramatically increases the speed and power of his attacks at the cost of his ability to guard or evade. Beyond his combat prowess, he is a brilliant strategist and an efficient administrator, likely handling much of the governance of Oushu. In quieter moments, he is also known to enjoy and be skilled at gardening and farming, a stark contrast to his fearsome reputation on the battlefield.