The Pacifistas are advanced humandrone weapons developed for the world government of the One Piece universe by Dr. Vegapunk. These cyborgs are based on Bartholomew Kuma—one of the Seven Warlords of the Sea—and generally represent a technological breakthrough in the world of the series.
The Pacifista Project was introduced prior to the two-year time skip in One Piece and marked the shift of power between the pirates and the world government. Each Pacifista has its outer skin made from the bodies of humans, but the original Kuma was an exception, as he received gradual cybernetic implants to make him into a Pacifista. This process of creation adds a dark tint to their existence, further raising questions within the narrative.
Physically, they are nearly duplicates of Bartholomew Kuma in terms of size and even don the same attire as him, where they have his black jacket, pants, and even his paw-pad gloves. What mainly sets them apart is that they can fire laser beams from their mouths and hands since they utilizes Admiral Kizaru's Pika Pika no Mi powers.
Each Pacifista is assigned a code, usually "PX" followed by a number. The Pacifista that the Straw Hat Pirates encountered in the Sabaody Archipelago was designated PX-4. Thus, this naming convention marks their identity with regard to each unit, and therefore refers to the mass production method applied in making these creatures.
They are immensely powerful, with abilities including superhuman strength and durability, which allow them to take massive amounts of damage. Their bodies are harder than steel; therefore, they are almost immune to conventional attacks. In addition to being able to fire laser attacks, the Pacifistas are also equipped with optical sensors allowing them to gather and process visual information to locate targets based on their appearance from the given bounty information.
Well, the etymology of the name "Pacifist" is rather depressing. Initially, it had been thought to be derived from their role in "pacifying" rebels against the Navy, but it was revealed that it really originated from Kuma's core desire for peace. That adds more dimension not only to Kuma as a character but also to the entire narrative surrounding the Pacifistas.
The latest events in the plot of One Piece have revealed a new bloodline of Pacifistas called Seraphim. They appear to be child-sized versions with aspects reminiscent of the Shichibukai. Such an evolution of the Pacifista Project has just shown a continued technological development, and by association, an increased arms race in the world of One Piece.
The Pacifistas belong to a number of the big story arcs. Their introduction in Sabaody Archipelago showed that with overpowering strength, they destroyed the Straw Hat Pirates and sent all of them their separate ways to train for two years. This served as a major plot catalyst by having the characters want to become stronger due to the happenings that will face them in the New World.