The Yomi Yomi no Mi, or the Revive-Revive Fruit, is a Paramecia-type Devil Fruit in the One Piece world. It has been said to grant the consumer an extraordinary ability: one can be resurrected upon death. This special Devil Fruit first belongs to Brook, a musician of the Straw Hat Pirates, and has deeply characterized him as a character and through his storyline in the series.
This main ability of the Yomi Yomi no Mi allows the user to raise one's soul to such a degree that even after dying once, he can be revived. The resurrection ability is really what separates this Yomi Yomi no Mi from most other Devil Fruits in One Piece. What sets the Yomi Yomi no Mi off from all the other fruits that grant physical powers or instantaneous transformations is that its true potential can only be unlocked after the user has died once. This unique characteristic develops an element of suspense and mystery surrounding the fruit's powers, as its full effects are not immediately apparent upon consumption.
Brook represents a rather unusual way of manifesting the fruit's power. As his crew was killed, Brook's spirit wandered for an entire year until it found its body, only to find that there was nothing but bones and decaying flesh. This aspect endows Brook with the iconic skeleton look he bears on his shoulders, a result of the power of the fruit and his resurrection circumstances. This outcome shows that Devil Fruits are unpredictable and that the effects may be altered by other external factors.
Futher than the initial resurrection, the Yomi Yomi no Mi gives its user a lot of extra powers about the soul and afterlife. Brook showed the ability to let his spirit exit his body, which enables him to go through walls and possess other people's bodies. This allows Brook to have a spiritual dimension in combat and utility among the crew. Moreover, this fruit grants a form of functional immortality to him as Brook no longer ages and is incapable of dying from natural means; he can, however, be killed through external means.
The powers of the Yomi Yomi no Mi have also granted Brook a special connection to the land of the dead. He is apparently able to hear voices from ghosts and has a deep understanding of the nature of souls. By doing this, this feature of the power of the fruit really makes Brook's character complex, often setting him in the role of mediator between the world of the living and that of the dead in several story arcs.
One of the most interesting things about the Yomi Yomi no Mi is that it can be further developed and awakened. In the world of One Piece, there have been instances in which Devil Fruit users "awakened" their fruits, which sometimes opened up access to new and more powerful abilities. Many fans are speculating on what an awakened Yomi Yomi no Mi would offer—others' resurrection or commanding armies of the undead. While these are speculations, the aspect of awakening adds some bias to Brook's character development.
Of course, the nature of the Yomi Yomi no Mi definitely raises a lot of questions on how Devil Fruits work in One Piece. For example, many fans come up with speculations as to when and how the fruit respawns following its user's "immortality" reaching an end. All uncertainty only further adds mystery to the fruit and relates to many general arguments over the nature of the Devil Fruits in the series.
On the other hand, in terms of combat applications, while the powers of Yomi Yomi no Mi are not directly offensive, they still benefit Brook greatly. He can conduct furtive reconnaissance with his soul separated from his body for surprise attacks. In addition, his undead nature makes him immune to some kinds of damage and allows him to keep fighting without much difficulty even with an injury that might cripple a human.