Kensei Muguruma is a Shinigami who holds the position of Captain of the Ninth Division within the Gotei 13, the primary military body of the Soul Society. He is also a member of the Visored, a group of Shinigami who have acquired Hollow powers. Over a century before the main storyline, Kensei was already a Captain, leading the Ninth Division with Mashiro Kuna as his lieutenant. During this period, he was sent to investigate a series of mysterious disappearances of souls in the Rukongai district. In the course of this mission, he destroyed a giant Hollow and saved a young boy named Shuhei Hisagi, who would later become his lieutenant. Shortly after, Kensei and many of his squad members were ambushed and subjected to Hollowfication experiments by Sosuke Aizen, orchestrated with the betrayal of his own fifth seat, Kaname Tosen. After being exiled from the Soul Society, he lived in the human world with the other Visored, including their leader Shinji Hirako. Following the defeat of Aizen, Kensei was reinstated as the Captain of the Ninth Division.

In terms of personality, Kensei is characterized as a serious, decisive, and temperamental individual who is easily annoyed. He has a particularly low tolerance for immaturity and childish behavior, a trait most frequently tested by his former lieutenant Mashiro, whom he has noted has the uncanny ability to aggravate him even when she is asleep. Despite his gruff and aggressive exterior, which is especially evident in combat, he possesses a caring nature. His attempts at kindness, however, are often expressed in a blunt, drill-sergeant-like manner, as seen when he tried to cheer up a tearful young Shuhei Hisagi. His motivation is often tied to his role as a protector and combatant, and he has confessed that he is not mature enough to hold back in a fight, even against a child. His actions are driven by a sense of duty and a desire to face threats directly.

Throughout the story, Kensei plays a significant role in both his past and present incarnations. As a Visored in hiding, he and his comrades trained Ichigo Kurosaki to control his inner Hollow. He later joined the battle against Aizen's army of Arrancar in the Fake Karakura Town, using his abilities to destroy numerous Gillian-class Hollows. After the time-skip, he returns to his former position as Captain of the Ninth Division, now serving alongside his lieutenants Shuhei Hisagi and Mashiro Kuna. His key relationships include his bond with Mashiro, whom he finds infuriating but whose life he has saved; his profound impact on Shuhei Hisagi, who was so inspired by Kensei that he tattooed the number 69 on his own face; and his connection to Kaname Tosen, whose betrayal led to his Hollowfication.

Kensei possesses a range of notable spiritual abilities as a high-ranking Shinigami and Visored. His Zanpakuto is named Tachikaze, which translates to Earth-Severing Wind. Its release command is Blow it away, and in its Shikai state, the blade transforms from a standard katana into a combat knife. In this form, Tachikaze allows Kensei to manipulate wind and energy, enabling him to fire explosive energy blasts known as Bakudantsuki and create invisible air blades that cut and then detonate. He has also achieved Bankai, the final release of a Zanpakuto, named Tekken Tachikaze, or Iron Fist Earth-Severing Wind. This Bankai transforms his blade into a pair of large knuckle blades attached to his hands and arms, focusing all of his weapon's destructive explosive power into his punches. As a Visored, Kensei can summon a Hollow mask that resembles a flat hockey face guard with six eye slits. Wearing this mask vastly augments his strength, speed, and durability, granting him a significant power boost by supplementing his Shinigami abilities with Hollow powers.