Junketsu's inactive form is that of a white and blue sailor uniform. The design is noticeably more militaristic than normal uniforms, with a high collar, shoulder epaulettes and gold adornments along the sides. When active, Junketsu's two eyes are located on the neck lapel.
Little is known about Junketsu's personality as it has not spoken so far. However, like Senketsu, it has a strong craving for blood and violently latches itself onto Satsuki Kiryūin after she awakens it with her blood.
In episode 6, Junketsu is shown to be under heavy restraint when not in use by Satsuki. This, along with Satsuki's technique "Life Fiber Override", implies Junketsu's personality is more feral and demonic, in contrast to Senketsu's more caring and calm personality. As such, instead of bonding with Satsuki, Junketsu seems to have to be tamed instead, having its ferocious nature subdued by Satsuki's willpower.
Junketsu appears to require more blood than Senketsu. This is evidenced by the transformation sequence of both Kamui. Satsuki uses three needles to pierce her skin, while Ryūko uses only one small blade.
Junketsu was created by the Kiryūin family to be worn by their young daughter Satsuki Kiryūin when she came of age; Satsuki's father called Junketsu her "wedding dress". Some time afterwards, Junketsu was sealed away in Kiryūin Manor by Satsuki's mother. Under her orders, Junketsu was kept under tight guard from everyone, even Satsuki herself.
Faced with Ryūko Matoi and her own Kamui Senketsu, Satsuki goes to Kiryūin Manor to retrieve Junketsu against her mother's wishes. Awakened by Satsuki's blood, Junketsu becomes crazed and clings to her body, craving more. Through sheer willpower, Satsuki keeps Junketsu's incredible power under control. She wears Junketsu in her next confrontation with Ryūko, dwarfing her and Senketsu's combined strength as Ryūko had yet to tap into Senketsu's full power out of embarrassment of the Kamui's "exhibitionist" appearance. Even after Ryūko manages to overcome this, Junketsu's strength still beats out Senketsu's. Amused by Ryūko's determination, Satsuki challenges her to defeat all the students in her school to prove herself worthy of facing her and Junketsu's power again.
Later, Ragyō Kiryūin succeeds in defeating Satsuki and steals Junketsu from her, claiming that Satsuki is not truly wearing Junketsu and cannot unleash its full power. Ragyō then dons the Kamui herself and activates Life Fiber Override, increasing her capabilities to the point that she can easily sneak behind Ryūko and pull out her heart with her bare hands.
Some time later, during Ryūko's battle against Nui, she was forced to wear Junketsu while having many Life Fibers sewn in her skin by Nui, and her memories were changed by Ragyō, making her believe that she was treated nicely by her, thus turning her into a brainwashed loyal minion. However, with the help of Satsuki and Mako (who now wears Senketsu), Ryuko is able to break free of Junketsu's control, thus breaking free of Ragyo's brainwashing.