There's overarching plotline #1: saving a friend with a mystery disease. Why did they contract this disease? When did Haiji even make a friend, if he grew up spending all day doing assassin training with his father?
Also, Chiwa seems to be a rather moral person as well. She mentions that the Black Coin are "super-evil people" and that therefore she wasn't sure if she should tell Haiji. So, Haiji works in the underworld, but only saves good people, apparently? How does that work?
On a completely unrelated side note, I wonder how much of this chapter arose from tensions between Japan and China. As someone who speaks Chinese, it was quite off-putting how many racial caricatures are in here, from the Tangzhuang, the Mandarin queue, to the squinty eyes...