Let the speculations begin!

 Luna D:

I dunno who is gonna be big mouse, but i'm sure multiple ppl work for him, the mayor's wife, the guard, some prisoners...etc. He is definitely not ONE PERSON organization. 

For some reason i think he is someone close or previously met  CH, may be someone related to a client or an old colleague.  

I 100% agree with you there's no way Big Mouse would be able to pull off all those crimes if he was working on his own, we still don't know anything about CH family so it could possibly be a relative. OR maybe even his family organisation maybe he know who Big Mouse is because his family is involved.

I was reading the OP's post and I don't think DID is likely. Park ChangHo would be shown to have memory black outs. His wife and father in law would have known about these moments. So far they haven't given us such hints. 

There is a possibility that he really is Big Mouse based on his resilience. But would he try to get himself killed then?

The preview for episode 5 with him in a fancy suit and car is definitely confusing. It can be that he has successfully convinced people he is Big Mouse and based on that manages to take an exit as a VVIP to meet someone. So the preview can be a hoax.

Jerry  according to me has the strongest possibility of being Big Mouse.

The guard is also very suspicious. Could be working for Big Mouse.

 YumzQ:

After watching episode 5, I became suspicious of Prosecutor Choi. He could have given the  actual list to the Mayor so that Chang Ho can tell the correct names of the clients and told Gi Hoon that he gave the fake list.  Also he could have had the opportunity to poison the witness since he was the only one in the room. Him asking the witness to give fake testimony could be that he needs Chang Ho to be considerd guilty so that he can go free.

The mayor received the wrong list though, that's why he was in a rush to the prison. Of course that isn't enough to rule him out entirely, but the fact that so far he's played a very minor role and that he has a clear motivation makes me think the prosecutor is not BM. 

On a side note, what do you think the room leader (of PCH and Jerry's unit) said to the warden in the previous week's episode? That one is still bugging me.

Another crazy possibility. Big Mouth / Big Mouse is actually a top secret agent. It probably runs in their family, his father was also a top secret agent.

Even how he ended up marrying  Mi Ho was probably a long-term orchestration as part of his cover up.

We've known (and it happens in real life) how top secret agents, and even the government agencies themselves, do keep a stash of illegal funds so they don't have to go through the red tape when they need to conduct an operation.

There are two possibilities if this is the case:

1. It was a long-term plan to bring down Ji Hoon and his gang.
2. He was activated after the Mayor recruited him as their lawyer -- which is technically and possibly their plan to ensure he can get "inside" them without them learning the government is into them.

Now, he is trying to be very strict and focus to protect his wife, ready to sacrifice their relationship for the sake of their secret operation to bring down Ji Hoon and his gang. He even gave the Mayor a choice who he will side with.

After all, there is no possible way Big Mouth knew the correct list, knew flowers will be sent to Ji Hoon's wife, knew things happening outside unless he has a network.

Someone who is simply pretending to be an ordinary guy can not possibly have a network outside. In the first place, he won't be able to send a message to anyone without the Warden knowing it. It means he has a well established network inside and outside.

And "Big Mouse" is his secret agent name.

 IM YourOnlyOne:

Three more speculations.

-- If we are going to follow the poster, "Big Mouth" (Park Chang Ho) is "Tom" from "Tom and Jerry". While the "Big Mouse" is actually Go Mi Ho (the "Jerry" in "Tom and Jerry").

-- The Korean title is 빅마우스 which is "Big Mouse" in English. But the official English title is "Big Mouth". It was established that Park Chang Ho's nickname is "Big Mouth".

What if the Korean and English titles are actually clues? Two facets of the same person? 빅마우스 (Big Mouse) is Big Mouth. A play on words and translation?

-- Or maybe it is the combination of both? When two people get married they become "one". Following the design of the poster and the play on words/translation between 빅마우스 (Big Mouse) and Big Mouth (Korean and English titles), could these be clues that "Big Mouse" is both Park Chang Ho and Go Mi Ho? They are playing everyone?

Yeah those could be big clues. To me I still feel that Park Chang Ho is Big Mouse and he was acting all along to be incompetent, as of episode 5. After all, he was great against the 3 guys even before going to jail. Everything that was planted in his office and the mysterious/anonymous tips would have been done by himself and he went to prison on purpose to bring down the NR forum organization and avenge the murder. Also PCH as BM definitely has contacts inside (the prison guard) and outside the prison. As for Jerry, I think he's a cop undercover and that's why he's uncomfortable with how PCH is acting as BM.

The Mayor recruited PCH....but what if it was his (PCH/BM) plan all along? After all the Mayor said he got his name through a tip. 

Honestly, all hints point to Jerry..

the biggest is, who is always aware of what PCH needs.. specially the drug client list.. only Jerry knew about it..

and it is not a coincidence that he has a mouse tattoo..

BUT

it feels too obvious, unless this is by a director who is not really into mystery direction.. then i can understand.. that jerry is not a red herring but a sloppy mystery direction

I don't think it is Park  Changho.  In the first episode, the radio broadcast was speaking of Big mouse and he said to himself ''please be caught so I can defend you''. Why would he say that out of nowhere, while being alone, if he was big mouse?