I'm pretty convinced it is the assistant.
1) Because I was attracted to him at the start. And for some reason whenever it is a "side" character and not the main that I find myself attracted, they either die or end up the bad guy. It sucks. I think it is likely because they are normally written as a good noble person which attracts me to them, but ultimately made that way to either be sacrificed or a facade for a big betrayal reveal later on.
2) Like many here, I had put the back story brother as the assistant long ago, as soon as we learned that no one knew his current whereabouts, I said out-loud, because he is secretly standing right next to the main lead...as the assistant. But that didn't bother me back then because the backstory seemed to show the brother liking the main lead. So hiding his past to help the guy that had helped his sister wasn't necessarily a bad thing. BUT....I do not feel that way anymore....explain below.
3) The brooding locked in his room brother, seems too easy...though since mystery isn't the main focus of this show, the mystery element may end up under cooked. I feel like he is cleaning up the metal bullets and witnessing events and knows who the actual killer is as a way of trying to gather evidence. I think he is researching how the bullets are used and how the culprit is getting their equipment. I don't understand how he got involved...maybe just witnessing something on a random night escape of his room...or there is some sort of history which is why he didn't take his test and had a mental break years ago...and thus he is secretly investigating whatever happened. All just conjecture. But, him showing up at all the locations, and seeming to know all the targets (even the poor kitties) is a bit of headscratcher. EDIT: Maybe he is hired and working as the muscle?
As to explaining number2 and why I actually HATE the assistant now comes from his manipulation and seeming to actually want to control the main lead, not necessarily help him. He also exhibits signs of not having empathy for anything or anyone except the main lead. But I even question this.
In the beginning when the main lead was sick, fainting, not eating, sleeping etc. he seemed to really care about him; however, I now question if that wasn't actually how he wants him. He prefers our main lead as a man that is broken and suffering while he is the only person in his life. Kinda like a Misery by Stephen King situation. That he either actually wants everyone involved with his sister to suffer including the main lead or if the main lead is the only person he wants to succeed. I now question which is his motive.
Back when the injustice originally happened to our female main's daughter being kicked out, the assistant didn't care. He also actively was against our main lead leaving the special 7 student teaching lessons. Even though it was done through backhanded dealings, that meant nothing to him. It is the way it is and they needed to move on. He also helped persuade the main lead to stay and was happy when the main lead stopped caring about the injustice. This, was when I first started to dislike him, and weirdly stopped finding the actor that plays him attractive. That goodness was gone.
He is then again against and upset when he finds out the main lead had decided to secretly tutor our female lead's daughter. He was openly against it. And recommended it end on multiple occasions, again not caring that our main lead was eating again, happy again, and feeling as if he had fought against injustice and found a solution that benefited everyone. The assistant wanted him to walk away. Again, showing that he really isn't a good person. That he doesn't care how things affect anyone outside of him and the main lead. Everyone else is expendable. But also, pushed for the main lead to date others, and that he would find other restaraunts. In short. Let them rot, I can find "safer" ways for you to be happy that aren't threatening to our success.
Now, in the last episode (9) he was again happy when the tutoring was revealed but at the expense of the female main lead and her daughter. He also didn't want them to fight the rumor even legally and was against any move that protected the others. He also was against our main lead leaving the special 7 student study group yet again. He was upset that our main lead was threatening to do so. Lastly, he was happy when the main lead decided to stop the tutoring sessions and even happier when he agreed to continue the special class.
Considering he supposedly supports the main lead, and wants his happiness, then why is he against everything that has been making the main lead happy? Why does he not go the "extra mile" to secure and ensure that they are not caught and finding ways to fight back all the crap that is happening? And again, his lack of empathy, even though the female lead and her whole families life is turned into catastrophe and ruin, he walks into their restaurant as if a no-name delivery man and hands over the workbooks with a smile then turns and leaves without a single hint if sorrow, apology, or that he even remotely has spent the past MONTHS having interactions with him. He cares nothing. Has no attachment. And is happy to put them in his rear-view, no matter how much they are suffering because of the school his boss works for or his boss.
I get the study session is the premier event for his boss, and the whole female leads family is a detraction to possible success. But as someone who truly cares for people, and that truly loves his boss and wants the best for him, that has morals and is a good person that wont turn blindly from wrong doings his actions paint him as a pretty shitty guy who only fights for financial success and job security. His boss already had all of this, money, fame, the special classes, the school, and it was basically killing him. Yet the assistant only wants to keep that going.
You could say he is just righting what his family did to him. The sister put in her suicide note that she caused the teacher to get fired, and so he is insuring he is never fired again. But that shows again his lack of understanding what a person really needs, his lack of empathy, his tunnel vision, and shows sociopathic behavior. Everyone can suffer, as long as we succeed. This also supports him not caring about killing people that hinder this path...aka, being the secret killer.
If I end up wrong...which I don't think...I still won't like him in the end...for these reasons mentioned here.