Salomon kept himself as busy as possible so he would not dwell on the sacrifice of the Sisterhood's Artificial Humans.
He liked Catherine, that lively Artificial Human. There seemed to be a slight flaw in her personality programming; she always acted cheerful, then went on to do absurdly foolish things that left people torn between laughter and tears. Even though he regarded Catherine, with whom he was close, as a daughter, he could not openly display his grief like the girls of the Sisterhood. He could only fill that part of his emotions with work, study, and the next war.
Naturally, he knew of Catherine's performance in the Fimbulwinter Campaign. As a soul born from his own soul and the Philosopher's Stone, Catherine was innately influenced by the Holy Marks. Even if Catherine's body died, her Artificial Soul would roam the Sea of Souls beside the projection of his soul. He had conducted experiments himself. He could communicate with Catherine through spirit magic, and he could summon her through the fourth-circle spell Spirit Guardian. However, the length of Catherine's stay, even the strength of her spirit form, was related to the duration and scale of the Holy Marks' activation. Under normal circumstances, she could not remain in the Material Plane forever.
He had made this attempt because his earlier attempt to activate the spare body prepared for Catherine had failed. Catherine's soul could not be dragged back by the spells inscribed upon that spare body. This did not match his original expectations. He had not anticipated that his own soul would exert such a powerful pull on Artificial Humans. Without activating the Holy Marks, the only way to keep Catherine's soul present for an extended period was to use the Philosopher's Stone.
He had already given that fragment of Aether