Cinnabar is a Gem from the anime Land of the Lustrous, distinguished by a unique and isolating ability to generate a powerful, mercury-like poison from their body. This poison is highly corrosive and harmful to other Gems, capable of ruining anything it touches and requiring any afflicted body parts to be completely scraped away. Due to this uncontrollable emission, Cinnabar lives in a state of profound isolation, both self-imposed and enforced by the community. They are deeply ashamed of their own nature, despise being seen by others, and have come to hate fighting, a stark contrast to their prodigious, yet unusable, combat talent which other Gems like Rutile acknowledge as extraordinary.

To mitigate the danger they pose to others, Cinnabar has taken on the solitary duty of night patrol, a role they devised themselves. Under the cover of darkness, the faint luminescence of their silver poison gathers available light, a property that helps them stay alert during these long, lonely watches. When not patrolling, Cinnabar spends their days isolated at a location known as the Hollow Cape, harboring a passive wish to be taken by the Lunarians, believing they might finally be of some use to someone. They possess a significant amount of knowledge about the world, knowledge that other Gems do not use or have forgotten, which leads their peers to recommend them to assist Phosphophyllite in creating a natural history encyclopedia.

Cinnabar's primary motivation stems from a deep longing for purpose and connection, specifically to escape the solitary night duty and find a meaningful role that does not endanger others. This desire makes them envious of the seemingly wanted protagonist, Phosphophyllite, who is actively pursued by the Lunarians. The core of Cinnabar's story is their relationship with Phosphophyllite. Initially dismissive of the brittle and clumsy Phos, Cinnabar becomes a catalyst for their promise when Phos accidentally learns their secret. In response, Phos makes a vow to find Cinnabar a better, more enjoyable job that is uniquely their own, a promise that gives Cinnabar a fragile sense of hope. Their dynamic is complex; while Cinnabar considers Phos a disappointment and finds their optimism pointless, they secretly cling to this one guarantee and are the only one who refrains from calling them a liar.

Within the wider community, Cinnabar's relationships are defined by their toxic nature. Most Gems, like Benitoite, find it impossible to relax near them, leading to a general atmosphere of awkward avoidance. Rutile is a notable exception, showing care by volunteering to deliver winter clothes and visiting when Cinnabar is injured. The Gem known as Diamond is also friendly, as they were born in the same year and can interpret Cinnabar’s reserved speech, a trait that tends to annoy the isolated Gem. Their creator, Adamant-sensei, cares for Cinnabar deeply, consistently reassuring them that their existence alone holds value, even if he cannot yet find a solution to their plight.

The development of Cinnabar is tragic and central to the narrative's later stages. The memory of Phos's promise becomes a powerful anchor. In a significant confrontation, Cinnabar fights Phosphophyllite, who has undergone immense transformation. At the conclusion of this battle, with their body shattered and impaled, Cinnabar expresses a final, poignant gratitude. They thank Phos for giving them a reason to be with the others, acknowledging that the promise was fulfilled by simply giving them purpose, right before they shatter completely. This moment underscores their entire arc: a journey from isolated self-loathing to finding a sense of belonging and fulfillment, not in a new job, but in the human connection offered by a single, kept promise. Cinnabar’s notable abilities include the generation of their corrosive mercury-like poison, which is so potent it remains effective and continues to cause damage even after their shattering, as well as their high-level combat skills, extensive knowledge, and the unique property of their poison to gather faint light.