What if the story of "Big Mouth" (en) / 빅마우스 ("Big Mouse" in Korean) was based on "Tom and Jerry"? This is like the Korean crime version of "Tom and Jerry".
More speculations inside.
Speculation #1: If we are going to follow the poster, "Big Mouth" (Park Chang Ho) is Tom. While the "Big Mouse" is actually Go Mi Ho (the "Jerry" in "Tom and Jerry").
Speculation #2: 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 the English titles are actually clues? Two facets of the same person? 빅마우스 (Big Mouse) is Big Mouth. A play on words and translation?
Speculation #3: 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?
Speculation #4: Park Chang Ho is a top secret agent of a black ops program. They setup a long-term plan to bring down Gong Ji Hoon and his gang. It would also explain why Park Chang Ho offered the Mayor a chance to choose a side.
Speculation #5: Park Chang Ho is indeed "Big Mouse" but decided to stay away from Gong Ji Hoon and his gang after he stole their money. Since the Mayor hired him as the incompetent lawyer, he had no choice but to get back to his old way to survive and protect his wife.
In both #4 and #5 it would explain why he was confident about the list of names, that nothing will come out of the witness, and knew flowers will arrive at Gong Ji Hoon's wife and call him at that very moment. Not to mention, he has information from the outside.
It would also explain why Gong Ji Hoon and the gang refers to "Big Mouse" as a person, and how Big Mouse's organization is everywhere (one such scene was when the three musketeers were asking Ji Hoon to transfer them to another prison). "Big Mouse" is a person, not an organization, he probably is the head of an organization (maybe a criminal organization or a black ops team).
Speculation #6: Dissociative Identity Disorder (Multiple Personality Disorder). Park Chang Ho's two identities probably already merged and it is only at this moment when he started to access the "Big Mouse" identity. Which would probably explain why "Big Mouse" disappeared.
Speculation #7: The "Jerry" character is literally "Jerry" the [Big] Mouse in this Korean crime version of "Tom and Jerry"; while the "Tom" is Park Chang Ho the "Big Mouth".
