Mathias first appears in Regnum, where she is pursuing Illia Animi. As she confronts Illia and Ruca Milda, Illia claims she does not know where the Manifest is located, but Mathias assures her that she will remember. When Ruca states that he is the reincarnation of Asura, Mathias laughs hysterically, which angers Ruca. Mathias leaves shortly thereafter, leaving Illia to explain to Ruca that Mathias attacked her village, leading a group known as Arca. She states that Mathias is pursuing her in order to discover where the Manifest is located. Subsequent mentions of Mathias describe her role in Arca, which is attempting to overthrow the Church.