Fosfofilita, often referred to as Phos, is the main protagonist of the story and holds the distinction of being the youngest among the gem characters. With a Mohs hardness rating of only 3.5, Phos possesses the most fragile and brittle constitution of all the gems, making the body entirely unsuited for combat. For the first three hundred years of existence, Phos was largely protected by the other gems due to this delicate nature and was considered clumsy and ineffective at most tasks.

At the outset, Phos displays a personality that is cheerful, naive, and somewhat immature, driven by grand aspirations to fight alongside the others and prove useful despite physical weakness. This initial desire to be a warrior leads to reckless, thoughtless actions that often create difficulties for those around Phos. Because of an inability to contribute to battles, Phos is given a first real assignment by the master, Kongou: to compile a natural history encyclopedia. Although this task is accepted reluctantly, it sets Phos on a transformative journey.

The drive to find a meaningful role for both oneself and for Cinnabar, a lonely gem relegated to patrolling at night due to a corrosive body, serves as a core motivation that fuels many of Phos's early decisions. This initial goal of improving the lives of outcasts eventually evolves into a much deeper and more obsessive quest for the truth about the world, the Lunarians, and the secrets kept by Kongou. Following a significant mental shock and the discovery of Kongou's secret, Phos plans a rebellion and cooperates with the Lunarian prince in an attempt to find a solution for all gems.

The character undergoes a dramatic and tragic development arc that is physically and psychologically tied to the body. Over the course of the story, Phos loses and replaces multiple parts of the original form. After losing the legs, they are replaced with agate and shell materials, granting high-speed mobility. When the arms are replaced with a gold-platinum alloy, Phos's personality becomes noticeably more subdued, calm, and at times, melancholic. The most profound change occurs after the head is replaced with that of Lapis Lazuli, a gem known for high intelligence and a calculating nature. Following this replacement, Phos largely inherits Lapis's wisdom, strategic thinking, and a tendency toward rational manipulation, becoming far more cunning and able to sacrifice individuals for what is perceived as the greater good. This series of replacements comes at a steep cost, as Phos loses significant portions of memory and eventually suffers from a strange insomnia that prevents sleep.

Several key relationships define this journey. The bond with Antarcticite, a partner and mentor during the winter season, is crucial; Antarcticite's shattering and capture by the Lunarians is a traumatic event that profoundly shocks Phos and contributes to a descent into a more压抑, or pathologically suppressed, state. The relationship with the other gems is complex; they are generally kind but do not initially consider Phos very popular or reliable. However, following the extensive body modifications, Phos becomes an object of curiosity and gains a different kind of attention from the others.

Notably, Phos possesses a unique biological ability that allows for this extensive transformation. The inclusions, the symbiotic microorganisms that constitute a gem's consciousness and identity, are exceptionally adaptable. Unlike other gems who would reject foreign materials, Phos's inclusions can adapt to nearly any new substance without a rejection reaction, even pushing the new limbs to perform at their absolute limits. Furthermore, Phos develops the unique ability to understand the speech of other lifeforms, including the Admirabilis, the snail-like creatures, and even the sounds of floating drift ice, a skill no other gem possesses. This communication ability allows Phos to learn secrets about the world that remain hidden from the others.