Borg is the collective term for a species of symbiotic alien organisms in the anime Alien Nine. Physically, a Borg resembles a brightly colored, winged frog with a textured underside that attaches to its host's head. It is worn like a helmet, using its wing-like appendages to hold onto the wearer's ears. A Borg connects directly to its host's cerebral cortex, which allows for a deep and intimate link between their consciousnesses. They communicate verbally with their hosts and possess distinct personalities.

The relationship between a Borg and its host is symbiotic, but it has unsettling elements. A Borg sustains itself by consuming its host's metabolic waste, specifically their sweat, which it prefers to obtain by licking the host's bare back. This physical intimacy underscores the invasive and personal nature of the bond. Despite this discomfort, the Borg's primary function is to protect its host from danger, particularly from other hostile aliens that invade the school grounds. Its primary weapon is the ability to project sharp, drill-like metal screws or cables from its body to attack threats.

A Borg's personality is heavily influenced by its host. The three Borgs assigned to the main characters each mirror the traits of the girl they are bonded with. The Borg partnered with the anxious and reluctant Yuri Otani is sarcastic, cynical, and unpredictable. It often tries to "mother" Yuri by taking over tasks but also insults her for being a baby, and its own reactions become confused by Yuri's uncontrolled fear. In contrast, the Borg belonging to the confident and capable Kumi Kawamura is wise, reliable, and serious, acting as a source of calm encouragement. The Borg of the cheerful but reserved Kasumi Tomine is more feminine in appearance and demeanor, reflecting its host's quiet efficiency and control. This mirroring shows that the Borgs are not merely tools but are dynamic partners whose identities become intertwined with those of their hosts.

The Borgs' role in the story goes far beyond simple protection. They are the central element of the school's Alien Party program, which tasks elementary school girls with capturing aliens. However, the program has a hidden and more disturbing agenda: to facilitate a complete and permanent fusion between the human host and the Borg symbiote. The Borg is the mechanism for this transformation. The process of mingling consciousnesses can have ill effects, and the ultimate goal is for the host to lose their individual humanity and become a seamless, unified being with their Borg. This is hinted at by the behavior of the teacher, who is implied to have already undergone this fusion herself.

The development of the Borgs is directly tied to the psychological state of their hosts. While a Borg's personality is relatively static, its behavior evolves as its host changes. For example, Yuri's Borg acts erratically because Yuri cannot control her emotions, but in times of extreme stress, its protective instincts override everything, sometimes with lethal force. The darker implications of the symbiotic bond are seen when a host breaks down completely. In the source material, a Borg can act as a backup for its partner's memories and personality, allowing for a disturbing form of reconstruction or rebirth after death. Ultimately, the Borg is not just a piece of equipment but a catalyst for the body horror and psychological transformation at the heart of the story, as the girls are slowly pushed toward a fate where they are no longer fully human.