
I think the thing that Kagami asked from his big bro is to watch Shingo for him, like when a new guy shows up -- shoo him away (something like that xD). As for the "mystery guy" from chapter 8, he maybe an important person from Shingo's past or he's the first guy Shingo fell in love with but it's an unrequited love ╮( ̄▽ ̄)╭
The chapter 2 of volume 7 explains why Naoya-san always look so serious when talking to Akira. Hahaha