Zekkai (絶界, Absolute boundary) is an expert-level element of the Hazama-Ryu Kekkai technique that produces a powerful Kekkai which surrounds and shields the user. Though the form can vary depending on the user, each Zekkai tends to have the standard appearance of a dark, flame-like aura. An incomplete Zekkai has a dynamic cloak-like form, while a mature Zekkai takes the shape of a sphere. Zekkai have considerable offensive and defensive capabilities: it can corrosively destroy virtually any object foreign to the user, and can repel all but the strongest attacks. A Zekkai is created by a Kekkaishi's domination of negative feelings. So far, the few known Zekkai users have been male, likely because of the intense power required to create it.
Unnamed Male Kekkaishi's Zekkai
While Kaguro was still a human swordsman named Kuroda, he went on a bloody rampage, killing every talented swordsman he came across. He encountered an unnamed male Kekkaishi in the woods, and, sensing that Kuroda was well on his way to becoming a demon, the Kekkaishi attempted to destroy him, producing a large, spherical Zekkai around his own body. The sheer power of it convinces Kuroda to flee in terror, but the memory allows Kaguro to effectively counter Yoshimori's Zekkai (and recognize it as being incomplete) when they meet later. This sphere is generally recognized as being a full or matured Zekkai.
Though he initially has trouble producing or maintaining a Zekkai (likely because of inability to completely dominate his negative feelings, partially due to timely interference from Tokine), Yoshimori produces his first working Zekkai in a mock battle with the Night Troop's Sen Kagemiya, Shu Akitsu, and Dai Yaegashi. Though just a thin film around his body, the Zekkai destroys Sen's elongated claws with a mere touch. By the time that Yoshimori challenges Kaguro, he is able to produce and maintain a kind of Zekkai armor around his body. However, it is only strongest in the areas where he has focused the most power, leaving others vulnerable to attack. With continued use, Yoshimori gradually improves the effectiveness of his Zekkai, though he seems to have a greater natural capacity for producing Shinkai, probably due to the even greater power requirements. Yoshimori is the first to discover that Zekkai can even protect against attack from psychic avatars, which are usually rendered almost completely undetectable by the psychic's power.
Masamori's Zekkai is extremely potent, and he uses it as a trump card in many battles against high-level opponents. His Zekkai also takes the form of a dark sphere surrounding his body. He typically charges at an enemy and destroys a large portion of their body through direct contact with the Zekkai,, or uses it to defend against attacks that would otherwise cause him serious damage.