Senketsu is a sentient kamui, a powerful garment crafted entirely from Life Fibers. Created by Dr. Isshin Matoi (later Sōichirō Kiryuin) specifically for his daughter Ryuko Matoi, Senketsu incorporates Ryuko's DNA into his construction. This biological link allows only Ryuko to hear his voice and achieve full synchronization. Isshin hid Senketsu beneath his laboratory shortly before his death, where he remained dormant until Ryuko's blood accidentally reactivated him. Desperate to avoid dormancy, he forcefully attached himself to Ryuko, establishing their initial partnership. She named him "Senketsu," meaning "fresh blood," referencing the blood that awakened him.

Physically, Senketsu typically manifests as a navy-blue and red sailor uniform with a midriff-baring top connected to a short skirt by suspenders. A multicolored scarf functions as his left eye, while a black eyepatch with an X-mark covers his right eye. When separated from Ryuko, his upper and lower sections resemble jaws, emphasizing a mouth-like appearance. During synchronization, his form transforms: the scarf extends into wing-like crests over Ryuko's shoulders, his color scheme shifts to include armored elements with red stripes, and the outfit becomes more revealing. His ultimate form, Senketsu Kisaragi, features a yellow, white, and reddish-orange color scheme, a broken eyepatch revealing a second eye, and energy bands forming a skirt-like structure around Ryuko's waist.

Initially, Senketsu craved blood manically to sustain his activity and displayed aggressive tendencies. Over time, he evolved into a composed, analytical, and protective entity. He demonstrates genuine care and concern for Ryuko's safety, often described as having "fatherly" love. He expresses loneliness when separated from her and displays human-like emotions, including sentimentality and crying. He dislikes being hand-washed due to the pain it causes but finds ironing soothing. His personality includes a protective streak, refusing to allow others like Maiko to wield his power against Ryuko.

Senketsu grants Ryuko immense physical enhancements and durability. His power requires consuming her blood, initially in large quantities, though this need diminishes after achieving full Life Fiber Synchronization, which unlocks his full combat potential. He possesses multiple specialized combat forms: Senketsu Senjin covers him in bladed spikes and claws; Senketsu Shippu generates propulsion jets for flight; Senketsu Mubyoshi counters sound-based attacks; and combinations like Senketsu Senjin Shippu merge flight and blades. He also experiences involuntary transformations, such as a berserk state triggered by Ryuko's heightened blood temperature due to extreme rage. His unique ability, Sen-i-Soshitsu, allows evolution by absorbing Life Fibers from other garments. Like all kamui, he is vulnerable only to weapons specifically designed to sever Life Fibers, such as the Scissor Blades or Bakuzan katana. He can monitor Ryuko's physiological state, including vital signs and measurements.

Senketsu's story involves several phases. After awakening, he aided Ryuko in battles against Honnouji Academy's student council, though initial synchronization issues limited effectiveness. Overcoming Ryuko's embarrassment enabled full synchronization, significantly boosting their power. During a confrontation with Nui Harime, Ryuko's rage triggered a berserk transformation. Later, Ryuko's hesitation led to Senketsu being destroyed by Nui. His pieces were distributed among Honnouji students, though Ryuko retained his scarf. She retrieved most fragments during the Tri-City Schools Raid Trip, rebuilding him. After Ryuko learned her Life Fiber hybrid nature and initially rejected him, Senketsu persisted. Temporarily worn by Satsuki Kiryuin to combat a brainwashed Ryuko, he aided in her rescue. In the final battle against Ragyo Kiryuin, Senketsu realized his partial human hybrid status (due to Ryuko's DNA) allowed him to resist Ragyo's control. To pursue Ragyo into space, he absorbed the Life Fibers from all Goku Uniforms and Junketsu, achieving his ultimate Kisaragi form. Successfully countering Ragyo's control over global Life Fibers caused critical damage. Senketsu sacrificed himself, using his remaining power to protect Ryuko during atmospheric re-entry, disintegrating completely.

Senketsu's relationship with Ryuko evolved significantly. It began with friction due to his blood requirement and Ryuko's embarrassment over his appearance. Through shared battles and experiences, it deepened into mutual trust, loyalty, and a profound symbiotic bond. They openly declared their friendship after mutually protecting each other from threats. Senketsu consistently prioritized Ryuko's well-being, acting as her protector, advisor, and confidant. Their bond is characterized as transcending typical relationships, encompassing elements of friendship, family, and partnership.