Changers, described in Consider Phlebas, are a pan-human subspecies, interfertile with Culture humans, genetically engineered as a weapon in the distant past by an unknown species. They are capable of impersonating any humanoid being of similar size by restructuring their body to resemble the individual.
Changers have the ability to grow, shrink and mold their body as willed, changing everything from looks to actual muscles and bones, though most major changes - induced by a trance-like state - take several days. The impersonation is nearly perfect outwardly, making visual identification as an imposter almost impossible. They are even capable of modifying the genotype of the bulk of their cells to match that of another person through a virus based biological process, presumably maintaining their native genetic structure in protected stem cells for later renormalization.
Changers also have conscious control of most of their bodily functions - these include the ability to produce copious amounts of sweat or small quantities of acid on their skin (useful for close combat or escaping from bonds, respectively), while the ability to shape their bone structure also allows them to slip through bonds if they have enough time.
Changers also have various natural weapons, with their bite, spittle and nails containing poisons or acidic substances, and have perfected associated techniques in disguises, impersonation (psychological tactics as well as subconscious behaviour) and combat (assassination and self-defense) to improve their abilities.
Because of the threat they pose to most humanoid societies that depend on appearance as a means of identity, the Changers are an almost universally reviled species, and are usually killed where found. By the time of Culture novels, they were isolated to a single large asteroid, known as Heibohre?, where they lived in a clan system of paramilitary structure. Some left that world for various reasons, and by the time of the Idiran-Culture War, many were working for the Idirans, mostly out of a dislike for the Culture.