Mattias has medium-length brown hair and wears a blue coat with fur trim.
115 years prior to Octopath Traveler, Mattias, once called "Matthew," was a priest of the Sacred Flame in the Frostlands. The village was struck by lightning, starting a wildfire. Despite his fervent prayers to the Flame, countless lives were lost. This caused Matthew to question his faith and eventually reach a "revelation."
That same year, Matthew gained 100 years free from aging from Lyblac and established ties with Simeon. He sought to weaken the Sacred Flame and allow more of Galdera's power to exit the Gate of Finis, which would grant him more power.
Over time, he slowly cut his ties with the church and began using Galdera's power to bring about miracles, establishing a cult following as the Savior of Wispermill. He worked his way back into the church under a new identity, as the trader Mattias.
In the final step of his plan, Mattias murders Archbishop Josef with poison provided by the Obsidians. Upon the Archbishop's death, he plots take advantage of the next Flamebearer, Lianna, using the despair of her father's passing to manipulate her into acting as his vestal in order to bring Galdera's flame to Orsterra.