Responses
I think her character served the purpose, exploring her character more may complicate the story for no reason. The extent of contact between Soerin and Moo already made Kirin ask if Moo was interested romantically in Soerin in a way and there is no way would I want to spend my life with someone I doubt loved my passed away sibling. I think author did great with using the charater to the minimal but with max effectivity.
For such a heavy story, it's well-written to say the least. Kirin irritates me in the first 20 chapters or so but Moo slapped him hard with facts later and it's satisfying tbh. Both characters are flawed but it's nice to see they find comfort in each other. Also, I was hoping the sister was explored more, i think she's an interesting character that bounds the 2 MCs together.