Well actually, I think the story is quite simple for now, although I agree it was confusing at the beginning. There is an assasins organisation called nine songs (the black haired and silver haired guys are part of it), and the guy in brown clothes came looking for the organisation to take revenge for them killing his master, which led him to interact with the two MCs. Now he is trying to use this to dig deeper to find out who exactly killed his master and execute his revenge. That is what I gathered after reading the available chapters.
Okay. Now it makes sense. Thank you.
This story is so confusing but I keep following the updates, hoping it would make sense to me someday.