A gift from Shiki Taishou: a replica armed with spiritual power. Though Shiki Koshou is incapable of charm magic, it is in contact with Shiki Taishou at all times and can share all that it has seen and heard with him, so it's almost like you've still got Shiki Taishou with you on your journey.
A gift from Shiki Taishou — a replica that possesses sentience. With the departure of Kamuna Harunosuke, the replicas scattered throughout the Mystic Onmyou Chamber would deteriorate over time, becoming nothing but fragments of memories and Charm Magic. However, it is precisely because this is so that "Shiki Koshou" is most rare and valuable indeed. It may well be the last remaining replica that can maintain constant remote contact with Shiki Taishou as a mark of the latter's deep attachment to his friends.