Heidi is the designation given to the parasitic entity co-inhabiting the body of the protagonist, Jeong Su-in, in the Korean drama Parasyte: The Grey. Her creation is the result of a critical failure in her species' standard method of reproduction. When the larval parasite entered Su-in, her human host was already bleeding heavily from knife wounds sustained in a violent attack, leaving the host near death. To ensure its own survival and that of its dying host, the parasite healed Su-in's injuries but lacked the capacity to fully consume and take over her brain, a process most of its kind complete successfully. This incomplete parasitization resulted in a unique symbiotic state where two distinct consciousnesses, the human Su-in and the parasite Heidi, must share a single body.

In stark contrast to Su-in's sentimental and emotionally driven human personality, Heidi is defined by cool, tactical rationality, possessing a perspective devoid of emotion. She does not make decisions based on feelings such as fear, loyalty, or compassion. Instead, her primary and overriding motivation is the logic of self-preservation; her survival is contingent on Su-in's, so she acts exclusively to protect her vulnerable host body from harm. This focused pragmatism often leads her to assess situations in ways that clash with Su-in's more empathetic instincts. For example, when Su-in interprets her mother's abandonment as a sign of personal hatred, Heidi offers a detached explanation: her mother did not hate her child personally, but rather the painful memories associated with Su-in and an abusive family situation.

Due to the circumstances of their unusual bond, Heidi has distinct limitations that define her role in the narrative. Unlike the parasite Migi from the original Parasyte story, who could occupy a hand and communicate freely, Heidi can only maintain control of their shared body for approximately fifteen minutes at a time. When she activates her abilities, which manifest physically as sharp, powerful tentacles extending from the side of Su-in's face often accompanied by a blackened right eye, Su-in loses consciousness and has no memory of Heidi's actions. Conversely, when Su-in is awake and in control, Heidi remains dormant. Because of this forced separation, direct communication is challenging and occurs either through short, written messages in a notebook or within a special subconscious space where the two can meet and speak.

As a creature who does not belong in human society but who is also rejected by her own kind for her failed parasitization, Heidi occupies a unique and hostile position. Because she is forced to follow the will of her human host, who often chooses to protect other people, other full parasites view Heidi as a dangerous anomaly and a traitor to their species. This isolation forces Heidi into a reluctant alliance with Su-in and a man named Seol Kang-woo, who becomes a crucial intermediary and protector for the pair. While Heidi cannot feel emotion, her existence within Su-in allows her to observe the positive aspects of humanity, including trust and resilience. Over the course of their journey, her cold logic leads her to an appreciation of Su-in's nature. In a final written message, Heidi acknowledges that while she continues to contemplate the meaning of their existence, it was a stroke of luck that she ended up in Su-in's body, reassuring her host that she will never be alone.