Due to being a male, he could not become an official member of the Demon Corps since men cant receive special powers after consuming Peaches and has instead become the caretaker of the Anti-Demon Corps 7th Unit, lending his support to his comrades both in and out of combat. He is a black-haired, dark brown-eyed young man of average height, with two prominent white hair strands from his center hairline, at his widows peak. His normal physique started being developed into a more toned one, as an aftereffect of regular training and acting as the 7th Units caretaker. On more than one occasion, Yuuki has been described as being either handsome or adorable by multiple women in the Anti-Demon Corps. His base state, under Kyoka, Yuuki assumes a smooth white-skinned muscular humanoid Shuki with a horned canine-esque face, with various red tribal markings across his back, shoulders, and chest; dressed in a pair of ballooning pants common to samurai, and black leather straps on his wrists. From the start, Yuuki has shown to be an idealistic, generous, and attentive person, who would assist his friends and allies if either of them is in need. He has issues with the current situation of the world where men are meant to be subservient to women because all of them can attain blessings from eating the peaches from Mato, but at the same time, he continues doing his duties. He has a strong desire to become a Hero and to protect people who are in danger, which has made him more devout to Kyouka, who was able to assist him in doing that, to the point of agreeing to become the 7th Units Caretaker. Since arriving at the 7th Unit, he has been somewhat socially awkward around its members, often never knowing how to interact with any of them because he has not been around many girls before. Despite his reservations, Yuuki does love working as a caretaker and housekeeper, in addition to assisting the 7th Unit, which stirred from his being taught such skills by his older sister during his childhood. Since childhood, Yuuki has grown up with his older sister, Aoba Wakura, who trained him to become a capable man (or someone to serve her in the future). She often referred to this as a video game and awarded Yuuki exceedingly high points whenever he did good jobs. Aoba also punished him whenever he did a bad job often using wrestling moves.
Although Yuuki is, in fact, human, upon becoming Kyokas Slave, he started assuming different forms bearing more of a resemblance to a Humanoid Shuki, wearing a collar linked to a chain, whenever her power is activated. Since then, he has assumed different forms based on either Learning and the "Lending" of her Slave depending on the particular woman in question.
In his Killer Fang form, Yuuki becomes extremely irrational and bloodthirsty, becoming a monster that only seeks to kill everything that stands in his way, whether friend or foe. Even Kyoka, who is normally stoic and fearless, was scared of this form and even advised canceling it instantly, noting its behavior was not anything like Yuuki. This is possibly due to Ren Yamashiros more ruthless and inhuman personality, in contrast to the other forms, where the masters have purer wills.
Six years before the events of the story, his sister vanished due to a "Mato Disaster" and he had not seen or heard anything from her since.