Who do you think is behind the Dog Mask?

I think that would be too obvious but definitely a good guess! I think he is somewhat involved but maybe not gaetal
I also think the teacher is an too obvious but he is definitely involved somehow and looks up to gaetal
the teacher's mom is very suspicious but tbh I can't really guess who gaetal is perhaps a less clear choice, someone from the police force...? anyways I am excited to watch weekly and figure out who it is bc there are so many theories

As of now (Ep3) I also think that Kim Moo Chan is the mastermind. But I have the theory that he is using the vote, and the helpers for it (maybe incl.  Gaetal), as a front to reach his actual goals. He was very insistent on the promotion  (for the mysterious investigation) as a reward of catching Gaetal.

- My Unserious Suspect will be Kim Jo Dan because.... Why not? No one suspect him so far I concern.. so He will be perfect Candidate and like a really good plot twist?, he got IT skill, there was these scene where he look at at Lee Min Soo at school Crime Scene, kinda weird.

- My Serious Suspects will be... could be anyone ?,

1.  Hope is  either Kim Moo Chan or Joo Hyun though because that will be cool but probably not, if it is, like I said it will be cool, because they are like Main Character, y'know like Mouse.  

2. Or like I said could be anyone but with Dissociative Identity Disorder and then that characters didn't even know that they are Gaetal.

3. Or characters that not yet even introduced or showed at all, but probably this will be weak as a story/plot wise

4. Or like people who read the webtoon, said it will be the student, my guess they were talking about Kim Ji Hoon (?) because I don't read the webtoon, so I don't know if it is the same character.. well this also weak.. for the one who knows, so my guess they change it (I hope)

- My Absolutely Not Suspect will be Lee Min Soo, because I just don't want to, if it is, my reaction will be like "Meh", second reason, my theory for him is to become the next/final target of Gaetal because I believed him to be the "Real" or second perpetrator to the murder of Kwon Seok Joo's Daughter. 

Hah, I knew it, The Teacher wasn't Gaetal (Almost Right, not confirmed yet) for being he was the Real Killer to KSJ's daughter murder from all the evidences that shown through out ep 1-4 (again, not confirmed, just based on evidences) and the "Killer" who got killed by KSJ probably innocent  because he had a mental disability therefore easily take blames for the teacher's doing and can be used over and over again on court by Assemblywoman to get easily win if police radar catch them.

Seems like she is and grandma and KJH are maybe plotting with KSJ. Something that maybe doesn’t mean anything but stays in my head is in episode 1 when KJH was buying like a thermo cup on the street. He was wearing a hoodie acting suspicious and paid for it with an envelope of cash. He bumbs into Joo Hyun and drops it and she apologizes and he leaves. Was something related to Gaetal in that cup? Any theories or clues in this scene or am I overthinking. 

 Rikuhearts79:

My theory on KJH bought a thermo cup was that, he want to be like the teacher (not what you think), y'know "to be like him", want to know what the teacher's thinking..

That doesn't make any sense to me. How would buying a thermo cup help him know what the teacher is thinking? Why doesn't he try his dressing style also then? That teacher is going to be Gaetal's target and KJH is going to enable that. At first, they will go after popular cases to get public sympathy and then it will get personal. Because the creepy teacher is the killer of KSJ's daughter. 

From a story POV, If KJH hadn't met that guy on that street then we wouldn't get our first meeting between him and JJoo and him realising that she is Joo Min's elder sister. Also buying a thermo cup in public is not suspicious at all, it is all very casual and explainable. However, I think the cup will appear again in a crime scene or somewhere related to Gaetal, and then JooHyun will recollect seeing it somewhere. 

After *Episode 5* I'm more convinced that KSJ is NOT directly involved with Gaetal and The Killing Vote.

He is most likely (or the case 8 years ago) the inciting incident, with everything stemming from his daughter's murder and then him killing the supposed murderer, but I don't think he is the mastermind in this situation.

His actions seem very opportunistic, and the directs hints and links by Gaetal (house as base, old delivery Guy, Trial speech, letters ...) seem more like nudges and jabs towards KSJ. Almost trying to force him to make moves, making everything even more obscure.