From the system archive, Thesus Yor was an orphaned teenage boy who lost his parents in a monster attack in their village, and he is the only one who survived that catastrophe.
With nothing left, he was about to end his own life when Rheina's father, the previous Clandestine Patriarch, passed by at that place and saw him. The Clandestine Patriarch took an interest in him because of how his eyes were so dull but still determined to kill himself as if it was nothing.
Because of that, he decided to take Thesus Yor under his wing and brought him back to the Clandestine clan to train him. Not only that, but the previous Clandestine Patriarch also trained Thesus Yor to become the right-hand man for Rheina, the clan's heiress and the next Headmaster of Clandestine clan and the next Leader of the Arcanum De Fleur.