Itsuki

"Drop ink on the finest snow-white paper and it will inevitably absorb into black, it's purity demands it! Tainted men have immunities, black does not absorb black. Sensui's change is but proof of his perfection and you can not stop him. The ability to prevent this demise would show purity does not exist. Purity is worth the world and worth it ending."

- Itsuki

Itsuki, code named Gatekeeper, is one of Shinobu Sensui's seven followers ready to help him end the world. Itsuki was the first demon to ever achieve any level of empathy with Sensui and ultimately what lead him to question his before-hand ironclad belief that all demons were evil and all human good.

Itauki and Sensui's past
According to Itsuki he had noticed Sensui while he was working as earth's spirit detective killing demons. Itsuki finds the lose of innocents literally erotic and Sensui had caught his eye as one of the purest people he had ever seen, the novelty of which made the spirit detective an obsession for Itsuki, who followed him from the shadows, occasionally protecting him fro other demons he found stalking him with less philosophical/erotic motives. Eventually Sensui caught-on to his stalker and proceeded to kill him, for no other reason than Itsuki was a demon. Itsuki was eventually cornered by Sensui with no place to run, when Sensui asked if Itsuki had any final requests Itsuki requested to live one final day so he could catch the final episode of  his favorite television show. Sensui laughed at the request, and for the first time stopped to talk to one of the demons he was sent to kill. Over time the two became friends and later lovers and Itsuki was even allowed to accompany Sensui in his duties as spirit detective.

Sensui and Itsuki were sent to disband the Black, Black Club, a group of human mobsters who were getting rich off demon-trafficking. Sensui and Itsuki were sent to a Black Black Club party where the entire group was said to show up, Sensi's mission would have dealt a critical blow to the demon-trafficking. The members had already left when the two arrived but the party had gone on. Sensui witnessed humans enslaving demons for both idol torture and sex, many cases of which had both at the same time. Sensui snapped and when Itsuki found Sensui he was covered in blood with everything, human and demon, in the room dead. Itsuki abandon his role as spirit detective and Itsuki left with him.

Sensui's return.
Sensui eventually returned to confront the spirit world with a plan to break down the barriers between the human and spirit world. While Itsuki found a site to act as a beacon for the demons and attempted to create a tear in reality for the demons to flood through Sensui's other six followers attempted to destabilize the city that was to be become ground zero. The chaos sewn also served to distract spirit world from seeking out Itsuki while he worked on the tear and allowed Sensui to size up the spirit world's new spirit detective Yusuke Yuramashi.

Sensui had recruited like-minded nihilist humans like himself as his followers though Itsuki is perhaps the only one to not hate man-kind, instead his motivations are based solely on is attraction to Sensui. By the time Yusuke and his friends Kuwabara, Kurama and Hiei make their way passed the other six followers Itsukji's has all but torn apart a portal to demon world and was taking a break since the space had been made weak enough for the demons on the other side were at that point steadily wearing it open. In an effort to give the two detective a climatic one on one battle Itsuki ordered his pet shadow demon who he had named, Uraotoko (Reverse Man), to swallow Yusuke's friends trapping them with him inside the creature's innards, which were it's own pocket dimension. During their imprisonment in the Uraotoko Itsuki reveals his and Sensui's past to the heroes. Itsuki warned them if they killed him they would be stuck in the Uraotoko forever ensuring he could not be harmed, instead Itsuki allowed Hiei, Kurama and Kuwabara to witness the fight thorough the Uraotoko's eyes. In truth the fight with Yusuke was a decoy, Sensui had suspected Kuwabara had the bower to cut through space and wished to use it to his advantage to open the barriers in place deeper in the portal allowing the bigger creatures in the depth of to break out. But Kuwabara had not tapped into his full powers so he had neither the ability or the will to open the spirit filter within the portal. To deal with the issue Sensui had planned to push Kuwabara as well as his friends to the emotional breaking point by killing Yusuke in front of them while trapped in the Uraotoko, a dimensional boundary that only Kuwabara's full powers could breach. Eventually Sensui did kill Yusuke and as Sensui had planned it motivated an drastic increase in Kuwabara and the others' powers, Kuwabara used his spirit sword to slice through the Uraotoko and the three chased Sensui into the demon-world portal ready to tear down the demon barrier to exact revenge.

Itsuki seems to be killed at first as the Uraotoko died but after Sensui is killed, Itsuki comes for his body and spirit. Itsuki was missing his right eye, which was the part of the Uraotoko slashed to tear it apart but was otherwise unharmed. Itsuki said that the battle was over and though they had won spirit world had no claim to Sensui as they had made him what he had become, a remorseless killer that eventually turned on them when he lost empathy with the humans. With no one in a condition to fight him, Itsuki took Sensui's body and spirit with him into one of the many infinite corners of limbo to spend the rest of eternity together.

Personality
Itsuki is the only semon other that Elder Toguro to follow Sensui. Itsuki is also the only member of the Sensui Seven that loves humanity, however Itsuki loves humans for the same reason all the others hate them, he finds humans capable of unspeakable evil. Itsuki literally gets off on seeing people who in their youth were pure lose their purity and become depraved, he likens watching a little girl who believes in babies coming from the stroke grow up to be a prostitute as being like porno to him. Though he fears for Sensui's life and does not wish to see him die or even kill the world he had never stopped Sensui instead wishing to see how far Sensui could fall from his previous station as a pure self-righteous demon hunter.