Les Serpos, also known as the Serpo aliens, are a fictional extraterrestrial species that appear as recurring antagonists in the anime Dan Da Dan. They originate from the planet Serpo, located in the Zeta Reticuli star system. The species is composed entirely of males, and their sole method of reproduction is through cloning, a biological limitation that has resulted in a complete halt to natural evolution and a profound narrowing of their emotional spectrum. In their true form, Les Serpos are slug-like humanoids with large, prominent lips and crystalline eyes that glow like beacons. When operating on Earth, they typically adopt the appearance of ordinary human males to blend in with human society, though their alien features occasionally shine through their disguises.

The primary motivation of Les Serpos is the advancement of their species through scientific means, specifically the acquisition of human reproductive capabilities. This goal drives their early actions in the story, as they attempt to abduct and experiment on human subjects to obtain the means for sexual reproduction, a process their own cloning-based biology cannot achieve. Their personality traits are shaped by their evolutionary stagnation: they display a marked lack of empathy and emotional depth, treating other beings as mere resources for their research. However, they are not without a certain unsettling curiosity about human culture and urban legends, suggesting a detached, analytical intelligence that finds human behavior both puzzling and useful.

In the narrative of Dan Da Dan, Les Serpos serve as the first major extraterrestrial threat encountered by the main protagonists, Momo Ayase and Ken Takakura, known as Okarun. Their abduction of Momo and subsequent confrontation with the pair triggers the central conflict of the early storyline, during which Momo discovers her latent psychokinetic powers. As the series progresses, Les Serpos reappear as a persistent antagonistic force, pursuing their research agenda in direct opposition to the protagonists. Their role expands from a simple initial encounter to a recurring menace that influences the development of the broader alien conflict in the series.

Key relationships within the group itself are defined by their cloning-based society, where individuality is suppressed in favor of collective goals. Offshoot clones that display deviation from the norm are considered defective. One notable member, Rokuro Serpo, a clone who assisted in fighting the alien Kur invasion, was subsequently branded as defective by his own kind and exiled. He later works at a convenience store, seeking alternative employment, and represents a rare instance of a Serpo developing a degree of independence and adaptation to human society. This character highlights the rigid, hierarchical nature of Les Serpos and their intolerance for nonconformity.

The development of Les Serpos as a group is relatively static; they do not undergo personal growth in the same way as the human protagonists. Instead, their evolution lies in their tactical adaptation and the gradual revelation of their society's limitations. Their most notable abilities include advanced cloning technology, the capacity to disguise themselves as humans, and a high degree of scientific knowledge, particularly in the field of biology and genetics. They also possess physical strength and resilience beyond that of an average human, though they are not invincible and can be overpowered by characters with supernatural abilities. Their primary strength, however, lies in their collective, systematic approach to problem-solving and their lack of emotional hesitation, which makes them cold and efficient adversaries.