Being 300 years old, they are the youngest among the gems. Their hardness is 3.5, low compared to most of the other gems.
They are brittle and weak so they are not suited for battle. Despite being clumsy and ineffective they were given the job of completing an encyclopedia.
They want to find new job for Cinnabar, who patrols at night and lives distantly from the other gems.
After being consumed by the sea snail Ventricosus, they gain the ability to understand her speech and decide to follow her into the ocean in hopes of finding a job that might suit Cinnabar.
Ventricosus subsequently offers Phos to the Lunarians in exchange for her brother Aculeasis. Phos loses both of their legs during their capture. Aculeasis escapes the Lunarians and gifts Phos pieces of his shell, which are imbued with agate. Phos’ legs are replaced by these, giving them incredible speed.
After the incident at sea, Kongou-sensei decides to cancel Phos’ encyclopedia job.
During winter they couldn’t sleep and did not join other gems in hibernation. Kongou-sensei allowed them to join Antarcticite in their duties, primarily consisting of upkeep and destroying ice floes. Phos lost both arms when they became distracted by the floes and fell into the water. Phos and Antarcticite search for a new suitable material to replace them, and try out a gold and platinum alloy.
After Antarcticite is taken by Lunarians, Phos is assigned to their previous tasks and helps Kongou-sensei throughout the rest of winter. They have difficulty sleeping, as they have nightmares about Antarcticite being shattered. Phos greatly matures over the winter and mimics some of Antarcticite’s looks and mannerisms.
Their new arms can become semi-fluid at will but are very heavy. As a result, Phos lost their previous speed.
In Spring, they temporary pair up with Bort. While on patrol they are attacked by an unusual type of Lunarian which later turns docile while in Kongou’s presence. Phos overhears Sensei calling the creature “Shiro” and begins doubting him.
Phos’ new goal is to uncover the truth about Sensei and the Lunarians.