The Yoko Yoko No Mi is a mythical Zoan-type Devil Fruit that transforms the person who consumes it into a Kitsune of Japanese mythology. "Yoko" (妖狐, Yōko) translates as "demon fox". In the Viz Manga and FUNimation dubs, it is called the Kitsune-Kitsune Fruit. It was eaten by Kurama.
Not much is known about the Yoko Yoko no Mi. It is said that among many rare Devil Fruits which were left behind by Gol D. Roger at Raftel, this particular fruit was one of them. It eventually found its way into the hands of Kurama, who was a Marine Captain at the time and eventually went on to achieve the rank of Admiral.
Kurama possesses the power of the Yoko Yoko no Mi, meaning "demon fox" (妖狐), which is a Mythical Zoan Devil Fruit. It essentially transforms the user into the mythological Kitsune, while subsequently granting said individual the powers and abilities associated to the creature. Having been acclaimed to have completely mastered the fruit's powers, at least on a fundamental level, Kurama can enter the transformation state attributed to the Zoan-type fruits on a whim with little to no indication of the shifting; allowing him to enter and release the form instantaneously. As a Kitsune Human, Kurama maintains a humanoid shape and form as his silver hair lengthens and darkens into a shade of illuminating lavender while his eyes and brow furrows into a matching set with similar coloration. In addition, he grows a long lavender tail from behind, which can be used as a fifth limb in close-quarters combat; in particular, Kurama is known to use his tail to strangle his victims or to hold them in place while he delivers a point-blank finishing blow. However, a complete transformation is not needed to invoke certain aspects of the Yoko Yoko no Mi's powers. For instance, only transforming his eyes allows him to use the hypnosis abilities of his fruit, while turning only his hair allows him to invoke elemental absorption. This demonstrates just how far Kurama has come to master his fruit's powers.
As the Kitsune is still technically a fox, the fruit bestows Kurama with the attributes of a fox. This includes an enhanced sense of smell and a sharpened hunter instinct which provide him with an edge in battle. It also grants Kurama an incredible increase in physical strength, speed, and durability. His strength is roughly four to five times that of his normal base strength, which make it possible for the Admiral to perform various impressive feats such as launching massive boulders with ease or smashing apart steel. What is perhaps Kurama's most impressive feat is that, while in this form, he managed to impose a significant crack to a considerable sized piece of Kairoseki before managing to actually smash through it with a following blow. Of course, this required some degree of effort on Kurama's part to accomplish. Kurama's speed is enhanced to a degree rivaling the concept of Soru, as he vanishes and reappears a vast distance away, manages to actually leave behind afterimages in his wake, or manages to escape narrow assaults with little space to move. His durability has also increased to a degree that he can take on a tremendous amount of damage without faltering enough to lose ground in a fight.
In classic Japanese mythology, the Kitsune are best known for their trickery and deceit. This is due to the fact that they possess an incredible ability to create lifelike illusions. Kurama has shown the use of illusions to often mislead his opponents and to throw them off to his actual presence. He is able to create a vast amount of illusionary clones completely indistinguishable from the real thing. Furthermore, he can even place others under a state of hypnosis, which is an extension of a Kitsune's power to deceive. In doing so, Kurama has amassed a rather significant amount of contacts around the world by manipulating certain individuals under his hypnosis and controlling them to do as he pleases. At first, his hypnosis caused those he placed under the technique to display a rather dazed expression while performing whatever he ordered them to do, but after training his skill and practicing it enough times, he can place a person under hypnosis and have them do as he wishes without the slightest change in their demeanor; almost as if each and every choice they make as a result of Kurama's influence is their own. What makes this even more frightening is that even the observational Haki cannot tell whether or not somebody is under hypnosis, but it can alert the user should they be targeted for hypnosis, allowing them to resist it to some degree. His skill with the supernatural deception skills of a Kitsune are proficient enough that he can even throw off a user of Haki with relative ease. However, that being said, an elite warrior with a sharpened Haki sense can still see through his illusions and even resist hypnosis, if not shatter it outright. However, this is also dependent on the victim's willpower in comparison to Kurama's own, which becomes the deciding factor.