Calling it: Kotsuga and Ryu are the same person.
The concept of a body detaching from a head was introduced- usually the body turns to dust if not attached, but we all know Kishi LOVES forbidden techniques. Maybe this samurai used a forbidden technique to split himself, hence Ryu having a two-soul weapon while Kotsuga has none. Kotsuga was the head, and retains all the memories and knowledge, Ryu is the body and retains no memories or knowledge but all the power. It's even reflected in how their outfits are designed.
The concept of a body detaching from a head was introduced- usually the body turns to dust if not attached, but we all know Kishi LOVES forbidden techniques. Maybe this samurai used a forbidden technique to split himself, hence Ryu having a two-soul weapon while Kotsuga has none. Kotsuga was the head, and retains all the memories and knowledge, Ryu is the body and retains no memories or knowledge but all the power. It's even reflected in how their outfits are designed.