it took me a bit to think about how he could slot into the story seamlessly like lee and yori do, but i think i finally know what i want to do with him plot-wise. in a nutshell a similar tragedy fell upon him like with yori where he also once protected a community fiercely and with pride, but—as it happens—it was also destroyed by sekishusai, being the first instance in which she demonstrated her battle prowess. unlike yori, however, shoyo was out of town at the town and came back to the entire place having simply been razed to the ground. the result of that was that he had nowhere to belong to and ended up isolating himself in the mountains, coping with what happened by gallivanting elsewhere and indulging in hedonism (not in a fat sense) in an attempt to avoid what happened. in the end his main arc is having his walls broken down so he's able to sincerely protect people again like with yori and lee, which is why he gets so upset when yori goes to the moon to pursue kumonryu as he viewed it as a personal failing that he couldn't protect her

so far as foils between himself and kumonryu are concerned, the both of them do have character arcs that concern the idea of "not taking responsibility", where shoyo simply refuses to acknowledge what happened through hedonism while kumonryu has no idea that what she's doing is wrong in the first place. as it happens the both of them end up having their worldview challenged by yori for different reasons; in shoyo's case, her conviction is enough to make him confront what happened and find something worth protecting again, while in kumonryu's case, yori's determination in coming to the moon to personally avenge the people she affected is enough to make kumonryu freak out and realise that her actions do have consequences regardless of the intention