Antarcticite, often called Antarc, is a gem whose very existence is tied to the changing seasons. Its unique composition causes it to remain in a liquid state for most of the year, only crystallizing into a solid, humanoid form when temperatures drop during winter. In this sense, Antarcticite is a being of the cold, growing stronger and more resilient as the temperature falls, while warmer weather would cause it to dissolve. This physiological trait isolates it from nearly all of its peers, as it is awake and active only during the harsh winter months when the other gems enter a state of hibernation.

Possessing a hardness of only 3 on the Mohs scale, Antarcticite is aware of its own fragility. Its personality reflects its solitary existence and wintry nature, initially coming across as stern, businesslike, and socially reserved. It is a diligent and almost pathologically hard worker, dedicated to its responsibilities as the sole caretaker of the community during winter. These duties include breaking up noisy ice floes in the surrounding sea, shoveling snow, and even putting other gems who sleepwalk back into their slumber. Despite its frosty exterior, Antarcticite is not mean-spirited. It hides a softer, more caring side that becomes apparent in its interactions, particularly with the protagonist, Phosphophyllite, who chooses to stay awake during winter. Antarcticite treats Phos like a slightly annoying but beloved younger sibling, patiently teaching them the ropes and taking responsibility for their well-being.

Antarcticite’s primary motivation is a deep-seated devotion to Kongo-sensei, the enigmatic leader of the gems. As Kongo is its only companion for centuries, his approval and affection hold immense significance for the solitary gem. Antarcticite pushes itself relentlessly to perform its duties beyond its best abilities, driven by a desire to earn praise and avoid disappointing its mentor. This drive is encapsulated in its own mantra, "Walk until you can't—then walk some more," which reveals a standard of self-sacrifice where weaknesses are seen not as limitations but as challenges to be overcome at any cost. This tenacity, while admirable, is also its tragic flaw, as it leads to reckless decisions and a refusal to retreat even when it is the safest option.

In the story, Antarcticite serves as a crucial partner and foil to Phosphophyllite. When Phos, burdened by guilt and a desire to become useful, opts out of hibernation, they are assigned to assist Antarcticite. Their partnership highlights Phos's insecurities, as Antarcticite’s effectiveness despite their own low hardness proves that fragility is not an excuse for inaction. Antarcticite’s role becomes a tragic turning point when it is captured by the Lunarians at the Shore of Nascency. In a desperate battle to protect Phos and prevent the loss of its own memories of Kongo, Antarcticite pushes itself past its limits, shattering completely and becoming easy prey for their abductors.

This loss is a devastating event for both Phos and Kongo, leaving them to mourn the winter gem and grapple with their own feelings of guilt. The development of Antarcticite is tragically brief but potent; it transitions from an isolated, duty-bound loner to a caring mentor and friend. Its relationship with Phos thaws its solitary nature, only for that newfound bond to lead to its undoing. Its last words implore Phos to keep Kongo from getting lonely during the winter, a final act of devotion.

Notable abilities of Antarcticite stem from its unique physiology and dedicated training. The colder the environment, the harder and more durable its body becomes, though it remains relatively brittle even at peak strength. It compensates for its low hardness with remarkable speed, agility, and skill. Its primary weapon is an obsidian sword with a serrated edge, more commonly used for breaking ice but also effective in combat against Lunarians. Antarcticite is also competent in basic self-repair, can identify structural weaknesses, and is proficient in the various chores required to maintain the school during winter, making it a surprisingly self-sufficient and capable gem despite its fragility.