"I told you, he and I were similar. When I was a human, I was an outsider just like Gen."
"When you were human..?"
"Although back in those days, I didn't have any superhuman talents like you two do. But still, I empathized and offered him some advice. I told him there was a way he could escape his torment. I suggested he cut his ties with this world in which no one accepts him. Just as I did."
-- Kaguro telling Yoshimori about how he was once human.
Kaguro wasn't always an ayakashi but a human born some few hundred years before the start of the series. His name was Genichiro Kuroda and he taught himself how to use and fight with a sword before joining a dojo. He quickly became rivals with another student named Sakai, and the two of them pushed each other into becoming the two best students in the dojo, but Kuroda could never beat him in their many spars. The day came when the two dueled to see who would take over as acting master of the dojo, and despite his training and skill, Kuroda lost to Sakai. Deeply angered, he went off on his own to contemplate how to get stronger, but couldn't think of anything other than amping up his training. Sakai found him and tried comforting Kuroda, telling him that he shouldn't worry about getting stronger because he's already an extremely skilled swordsman and that his greatest strength, which makes him a better fighter in Sakai's eyes, is the fact that he never hesitates and is always sure of himself. Sakai told Kuroda that the only reason he lost was probably because they were using practice swords and ventured that if Kuroda was using a real blade, he would have won.
Kuroda took this the wrong way, however, stole a real blade, and began challenging other skilled swordsmen around the land. He gained a sickening thrill and sense of accomplishment from killing other swordsmen and was soon wanted by many villages for his crimes. One night Kuroda came across a kekkaishi who sensed the evil within him and Kuroda immediately attacked. The kekkaishi was shocked by his level of strength with a blade and concluded that Kuroda had ceased being completely human and was now part ayakashi. The kekkaishi activated his zekkai, an advanced kekkai technique which forms a sphere of deadly energy around the user capable of disintegrating most anything that comes into contact with it, and prepared to fight Kuroda. Seeing how outmatched he was, Kuroda turned and fled, eventually stopping at a river to drink.
At the river, Kuroda met three ayakashi who told him that if he were to give himself over and become an one of them, he would have the strength to kill the kekkaishi. Before he could complete the process, Sakai found him and pleaded with him to reject the darkness and come back to the light as there was still hope for him. Kuroda told Sakai that he planned to seer all ties to this world in order to become stronger, and Sakai promised that as his only friend, he was bound by duty to end his suffering. The two friends drew their blades and began their fight. Sakai countered Kuroda's first strike and was poised to slice his head down the middle but hesitated, giving Kuroda the chance to slip his blade past Sakai's guard and cut his neck. With Sakai dying on the ground, Kuroda called out to the ayakashi to join their world, and was struck by a bolt of lightning in response. He rose as a full ayakashi and then traveled with the three away from Sakai, eventually changing his name to "Kaguro".
With the Kokuboro
"Fifty points. You really don't deserve more than that. However, given the fact that your abilities are still growing, I'll let you slide this time. I have high hopes for you. You can call me Kaguro. And, just for you, I'll leave for tonight."
-- Kaguro sparing Yoshimori during their first encounter.
Sometime after, Kaguro joined with the organization called the Kokuboro. Due to his set of powers, he quickly became one of the strongest ayakashi in the group and was capable of resisting Byaku's Parasitic Bugs, which kept all the other weaker ayakashi under control. Because of this, he was allowed more freedom around the castle than others and wouldn't even participate in any of the groups meetings. Due to unknown reasons, the Princess of the Kokuboro Castle became ill and needed a new source of power. Karasumori was the ideal choice, and soon efforts began to try and take the power of the land away from the Kekkaishi. The lead scientists, Aihi, began experimenting with human skin, intending to create a disguise for ayakashi to sneak into the grounds avoiding detection and even during the day. After one failed attempt, Kaguro volunteers to wear a human skin and take a team of ayakashi to infiltrate Karasumori.
Sanan, Haizen, Sekia, and Haroku are the four chosen to go with Kaguro, all equipped with a human skin. The enter Karasumori Academy during the day and make their way to the broadcast system, announcing themselves to the students and the Kekkaishi they knew went there, telling them to meet up at the school later that night. When night came around, Kaguro and his ayakashi group attempted to negotiate with the Kekkaishi, Yoshimori Sumimura and Tokine Yukimura, along with their comrade Gen Shishio, in order to allow them to surrender Karasumori peacefully so there wouldn't have to be any unnecessary fighting. Gen, due to his instincts and sense of smell, was able to figure out that Kaguro was the strongest and fastest of the group and attempted to charge ahead and take him out. Kaguro was unfazed, however, as he expertly sidestepped and slashed through Gen's shoulder, incapacitating him before anyone else could figure out what had happened.
Kaguro then quickly takes out Madarao and Hakubi. Yoshimori and Tokine's demon dogs, to prevent them from offering their help, and takes a swivel chair to the top of Karasumori Academy to watch the fight. Gen is taken away from the Kekkaishi while they are led to an area lined with special stones that with talismans that stop them from using their kekkai jutsu. As the fight rages on, Yoshimori and Tokine work together to escape the area and kill Haizen and Haroku, much to the disappointment of Kaguro. The final two ayakashi manage to push Yoshimori and Tokine and even take Tokine hostage when they get distracted by Gen's sudden surge of power. Backed into a corner, Yoshimori unwillingly activates his quasi-zekkai, which greatly amuses Kaguro as he now views Yoshimori to have the potential to be a great adversary. He makes his way down and slices through Sanan and kills him, freeing Tokine in the process and taking out the last member of his team. When asked about this by both Sanan and Yoshimori, Kaguro simply tells them that he couldn't stand how inelegantly Sanan handled the situation and that he hated the idea of winning like that. His human skin starts to peel off, signaling that it's at its time limit, so Kaguro bids Yoshimori farewell, telling him to get stronger so that he could have fun fighting him in the future, before leaving and heading back to the castle.
This seemed to be Kaguro's power once he transformed into an ayakashi. He could create sword blades from his hands that either stuck out of his palms or manifested themselves for him to grab and wield. These swords weren't your ordinary swords but ones that grew progressively stronger. As pointed out by Sen Kagemiya, Kaguro is a battle-type ayakashi and is geared towards fighting and nothing else. When Sen showed Yoshimori shards of Kaguro's broken swords, he dropped them on Yoshimori's kekkai and found out that the later pieces of sword fell farther through Yoshimori's kekkai than the first one, meaning that his swords only got stronger over the duration of a fight. Kaguro has also shown the ability to manifest swords out of the back of his arm up to his shoulder, creating another word immediately after one was broken, and even firing off small shards of his swords at a distance.
Anti-Regenerative Capabilities: Kaguro's swords have shown to cancel out altogether and severally slow down the regenerative properties of ayakashi and ayakashi-majiri (humans that are half ayakashi/have ayakashi powers). Hiba Kyouchi told Yoshimori and Tokine that Gen's wounds weren't healing because the cells around the injury were damaged themselves and weren't working properly. This is also seen in his fight with Aihi, where the branches that were cut by Kaguro didn't grow back when they used to almost immediately. This makes Kaguro especially effective against ayakashi-majiri and enemy ayakashi and extremely deadly.
Kaguro, although not a physically threatening opponent, has displayed levels of strength easily above superhuman standards. Even as a human, he was capable of cutting thick tree trunks in half and even a large bear with one slash of a katana. As an ayakashi, he's sliced right through a large ayakashi and a thick block of rock directly under him with one swing and sent Gen flying across a street with a nonchalant kick.
One of Kaguro's defining traits is his extreme speed. In his first encounter with Yoshimori, Tokine, and Gen, he was able to easily react to Gen's full speed charge to sidestep, draw a sword, slash him along the shoulder, and discard the sword before anybody could notice what happened. He's also danced around Gen's attacks when he went to see the boy during the day, and has repeatedly been too fast for Yoshimori to catch in a kekkai despite the latter training extensively to cast his jutsu faster. When he killed Gen, Kaguro was able to time Gen's attack to appear right behind him and stab him in the chest. That feat is made all the more impressive because Gen had fully transformed into an ayakashi, which greatly boosted his speed to the point where he was barely visible as a blur jumping around Gagin.
Although not specifically shown in the series, Kaguro has a healing factor/regenerative abilities, just like any other ayakashi.
As a human, Kaguro was a self-taught swordsman who achieved a level of skill equivalent to his friend Sakai in only a matter of two years. Sakai would have trained longer than that because he was a part of the dojo when Kaguro arrived. During his bloodlusted rampage, he sought out and challenged every skilled swordsman he could find and managed to kill everyone he faced to the point that he became a wanted criminal.
Kaguro has shown a very astute and observant mind throughout his battles. He took note of an recorded the strategies Yoshimori and Tokine used to defeat his group of ayakashi when they attacked Karasumori and filed that information away for any future fights against them. During his fight with Yoshimori, he was quick to call out the flaws in the kekkai jutsu and Yoshimori's fighting style, stating that he's limited by his eyesight and that if he can't react to or see his opponent, his techniques are useless. Kaguro was also able to discover a weakness in Yoshimori's quasi-zekkai by repeatedly attacking him over and over, finding that the layer of armor wasn't as equally concentrated around Yoshimori as it should have been.