Mathias leads the Religious Organization supporting the Knights of the Silver Tree in Arcadia. He is 28 years old and 180 cm tall, with distinctive mint green hair often tied in a loose side ponytail and light green or blue eyes. His position is later revealed as illegitimate; he was chosen solely for his exceptional memory skills after the previous leader died before the true successor, René, was born.

During his early life, Mathias underwent intensive training to assume leadership. Tasked with reading and memorizing an entire library of books, he completed this within seven years despite initial hesitation about its difficulty. He acts as mentor to René, who lives confined within the Holy City of Celestizam. Mathias hears René's frequent comparisons of himself to a "caged bird," reflecting their complex relationship. Initially, Mathias displays kindness and consideration toward others, earning reverence and trust through his compassionate demeanor.

In *Second Age*, Mathias's devotion to the Holy City intensifies, driving him to commit morally questionable acts. He kills the High Elder of the Religious Organization to eliminate threats to his authority and secure his position. Desperate to protect the city and retain his role, he implants an experimental artifact chip created by the scientist Erenfried into his neck. This chip grants him artificial purifying abilities but causes severe consequences: half his body becomes controlled by a Thanatos, corrupting his being and fracturing his identity.

His physical and mental deterioration escalates as the Thanatos influence grows. Mathias ultimately dies during an encounter with Nyx, fully possessed by the entity Erebos. In his final act, he sacrifices himself to shield René from Erebos's attack, succumbing to the fatal blow while attempting to protect his ward.