I feel like it's the assistant and he has crush on ML and he is the brother of that student girl

I def think it’s assistant ji I think he’s the girl who killed herselfs brother (, I think he got kind of obsessed with Mr chi after the funeral where he gave him his sisters pen ) that’s why when the detective (the older one)  saw him he said he looked familiar because he worked on the girls case himself. Also it wouldn’t make sense for it to be sun jaes brother because the people that die are always people who wronged mr chi but the brother doesn’t have access to this sort of information like what Mr jin was going to publish but it makes more sense if assistant ji is the one attacking because he would have access to this information, also when haeng seons store was attacked the only person that could know that her and Mr chi were fighting during that time is assistant ji. And the scene where he threw his phone after not being able to contact Mr chi kind of confirmed all this for me because he seemed to have a scary side to him.

 guyun:

SunJae´s brother but I guess it would have been too obvious. but I really hope it is not Ji DongHee because he is just amazing and I love him 

He's sooo creepy. 

 Shine Son:

Ji Dong Hee is CCY's assistant, confidant, secretary, driver and everything else... But not a friend. A friend can only be happy for a friend but by the way he is acting in the recent episodes... He is not. Do you see CCY considering him as a friend as well? I don't get that vibe.

However, these kinds of hints... The ones that make the viewers focus at one character as the "obvious" culprit can sometimes be used by writers to divert our attention from the actual culprit. This... Sun Jae's brother is one. Ji Dong Hee could be another.

It could also be possible that the culprit is a woman/girl.

You're off it mate. The most recent episodes and promos for the next episode, all but confirm that Ji is nuts and is actually the younger brother of the middle or high school girl that committed suicide and who was embroiled in the murder case of his own mother.

Personally, I like the show and I think it could have done without all the murder mystery but I guess it's there to cause friction for the leads.