Fosfolita, most often referred to as Phos, is the main protagonist of the story and is introduced as the youngest of the Gem characters. With a hardness of only three and a half, Phos is considered the most brittle and fragile Gem, making them entirely unsuited for combat against the Lunarians. As a result of this physical limitation and a general clumsiness that makes them ineffective at other practical tasks, Phos has spent the first three hundred years of their life without a formal assignment. This lack of purpose is a source of deep frustration, as Phos desperately desires to be useful and to contribute to the community.

Initially, Phos displays a personality that is naive, overconfident, and somewhat lazy. They are often scolded for talking back to their guardian, Kongo, and their cheerful, impulsive nature masks a growing inferiority complex born from being constantly labeled as useless by their peers. Their first official job, given by Kongo, is to compile a natural history encyclopedia, a task Phos finds boring and insignificant compared to the vital combat roles of other Gems. A key early motivation for Phos is encountered after meeting Cinnabar, a powerful but isolated Gem whose poisonous mercury forces them to work alone at night. Deeply moved by Cinnabar's loneliness and desire for a better role, Phos makes a solemn promise to find them a suitable and happy job.

Phos's role in the story is defined by a continuous and dramatic transformation, both physically and mentally. They are the only Gem whose body undergoes repeated and extensive replacements. After losing their legs in an incident with a giant snail-like creature named Ventricosus, they are given new legs made from agate and shell, which grant them incredible speed. Later, during winter duties, Phos loses both arms and they are replaced with a powerful gold and platinum alloy. These new arms can shift form and become semi-fluid at will, greatly enhancing their combat ability. The most significant replacement occurs after Phos loses their head to the Lunarians; the head of the brilliant but manipulative Lapis Lazuli is attached to their body. This event fundamentally alters their personality and intellect, granting them strategic thinking but also a more secretive and calculating nature.

Key relationships are the primary drivers of Phos's development. Their partnership with Antarcticite during winter is a turning point. Phos witnesses Antarcticite being taken by the Lunarians, an event that shatters their carefree spirit and leaves them with deep trauma and guilt. Phos cuts their hair short to resemble Antarcticite and carries their broken blade as a memorial. Following the loss of another partner, Ghost Quartz, Phos attempts suicide using their own alloy, an act that reveals the immense psychological toll of their existence. Their promise to Cinnabar becomes a distant memory as Phos grows more powerful and isolated, with the two drifting apart. On the moon, Phos forms a strained alliance with the Lunarian prince, Aechmea, who replaces their missing eye with a synthetic pearl to monitor them.

The character development of Phos is a tragic descent. The initial optimistic and clumsy child gradually becomes a being fueled by rage, despair, and a thirst for vengeance. After discovering secrets that Kongo has hidden, Phos betrays their former friends and leads a rebellion. Following a devastating confrontation where the other Gems shatter their body, Phos is buried for two hundred years. When they are finally reconstructed by Kongo, nearly all of their former kindness and warmth is gone, replaced by a pathological obsession to force Kongo to pray, no matter the cost. Their form becomes jagged and incomplete, with only half of their face remaining, and their alloy leaks from them uncontrollably, reflecting their broken mental state.

Notable abilities arise directly from the replacements made to their body. The agate legs provide high-speed mobility. The gold-platinum alloy arms are highly versatile, capable of hardening into weapons, stretching, and forming defensive shapes; the alloy is semi-sentient and responds to Phos's emotions, even shedding like tears when they are sad. After receiving Lapis Lazuli's head, Phos gains heightened intelligence and a talent for strategy and manipulation. They also possess a unique constitution where the microscopic organisms, or inclusions, that animate their body are highly adaptable, allowing them to accept and integrate foreign materials that would be rejected by any other Gem, though this process slowly causes them to lose memories of their past self.