Don't you wish he would realize that Si won is his dead best friend?
I think he's suspected that he is since the first time he beat Si Won up.
Hints:
Blondie admits that he is crazy (I think he is referring to his suspicion that Si Won is Tae-Min). He knows that that this is unlikely/unrealistic, but because he is in mourning, he is willing to labele himself crazy, if being crazy means that his friend is still alive somehow.
Blondie's friend mentions that Blondie has been lifeless recently (probably due to Tae-Min's death) and all of a sudden he's come alive again/taken interest in something. He wouldn't take interest in Si Won if he didn't have a hunch.
I knew it was harada as soon as I saw the cover, that creepy grin gave it away Sensei I have never once regretted opening the chapter knowing it's you.....XD