Let the speculations begin!

His friend I think because Big muse has to be someone close to him close enough to get into his office to plant the evidence or the person sneak in.

Or like you said Park Chang Ho has Dissociative Identity Disorder.

His punch is strong, coz he is a former athlete after all and he was angry and desperate, Adrenaline is a very powerful drug. I don't think he is Big mouse, but someone close to him is or at least close to Big Mouse, might be his father in law or lawyer friend or even Jerry his cellmate.

I feel like DID is a pretty big stretch because it's not like Big Mouse was a serial killer or a criminal operating on a local scale. Big Mouse was an international criminal, infamously so, which takes a lot of time and effort. Like unless Changho was blacking out for prolonged periods of time (probably days or weeks on end) and just ignoring it, there's no way he wouldn't be aware of it. Not to mention his relatives and friends. This isn't something he could just go out and do on his off hours, its literally career criminality. 

Also, why would his split personality hide things in his own office - feels like something very stupid for a genius swindler to do. 

hmm after watching ep 2 i also think its not ji hoon , mayor, or his wife. The question is why did someone frame our main character as Big Mouse. Who gets the most profit by framing him and giving the gold and money to the police???  And if he is the big mouse than who gave the tip that PCH is Big mouse??

Now this might sound very absurd but i think JErry might be Big Mouse. I mean we know that Big mouse is very cunning/smart and all. So who in their right mind would think that Big mouse is actually in jail for some minor charges(this is just a guess idk his charges). But then who framed PCH and why? idk maybe their is a different story which will come out as episodes go.

The reason why i dont think its PCH is because of his actions so far. He seems like a genuine guy who is actually depressed because of this situation. I really dont think its PCH. But i really don't get how they got the money and drugs from his house. Like seriously which dumbf*ck would think of hiding all these illegal things in their own house. Maybe the prosecutor or mayor are trying to do something here. There is a possiblity that his FIL is bad but i really hope that's not the case here.

Jerry would def be interesting, but I think we need more info on when Jerry (or the other suspicious inmates) got to jail versus when Big Mouse stole money from LaLa Capital/Gong Ji Hoon. It seemed like that occurrence was relatively recent, while Jerry and the other inmates have obviously been in jail for a while. So for the moment, my bet is still on someone on the outside. However, since we don't yet have a clear timeline of Big Mouse's last discernible crime, this could change.

 starflakes:

I feel like DID is a pretty big stretch because it's not like Big Mouse was a serial killer or a criminal operating on a local scale. Big Mouse was an international criminal, infamously so, which takes a lot of time and effort. Like unless Changho was blacking out for prolonged periods of time (probably days or weeks on end) and just ignoring it, there's no way he wouldn't be aware of it. Not to mention his relatives and friends. This isn't something he could just go out and do on his off hours, its literally career criminality. 

Also, why would his split personality hide things in his own office - feels like something very stupid for a genius swindler to do. 

I know right why would a famous criminal hide his wrongdoings in his office? 

The gold bars they found in his office in a briefcase look like the same gold bars in the opening of ep 1 that someone buried.

Oh dear, I am not suspecting his father-in-law or way be his wife.

His wife is not so much, however, you can never be too sure because the bad guys are always close and have anyone ever suspected the mayor when he met him I thought he was not the mayor and I have a weird feeling.

To find out who is big mouse we must first look at instants in which he appeared so far:

-at the beginning of the series where he is burying gold bars this is probably the money stolen from gong hi Hoon the newspaper boss.

-when Chang ho and the mayor when after the wife of one of the three prisoners who had the dash cam recording, he hit the women on forehead and left her alive for some reason even tho it could reveal his identity later.

- after Chang ho accident, the big mouth or probably someone else went to get the dash cam recording from his wrecked car.

Conclusion: 

The mayor and the big mouse were working together at some point but the mayor doesn't know anything about big mouse himself. Apparently big mouse said something disturbing about the mayor to the prisoner wife who had the dashcam as she looked at him in distraught.

Why would the mayor work with the big mouse. They are both in similar situations; the mayor wanted to cut off those who helped him reach his position as they keep blackmailing him, big mouse wanted to push all his crimes onto innocent change ho to stop the hunt over him( a special team was made to catch him).

When Chang ho asked for the mayor to visit him before we was sent to prison, the mayor told him that even though it was his first offense ( drinking while driving) the newspaper boss can easily send him to prison, he probably wanted to twist around the facts to confuse Chang ho and make them fight among each other. While the mayor work in coordination with big mouse.

Conclusion: 

Big mouse is someone who knew Chong ho and is close to him as the mayor specifically called him among all layers (being with the least success rate isn't convincing enough) they could have went for someone else with lots of depts and could be baught with money to stage up a play.

Big mouse is someone who knew that the prisoner wife had the dash cam recording and went after her before the mayor and Chang ho.

The only people who knew about that is Chang ho, the mayor and finally soon tae ( Chong ho's friend) who Deviced the whole plan.

Currently soon tae is working with Mi Ho and the father in law to find out this paper that was talked about in the dash cam if my guess is correct it's about how the mayor reached his position or something that the newspaper boss used to blackmail him.

If in next episodes soon tae tried to wiggle his way between our main couple this should solidify this theory.

I looked the big mouse body frame numberous times he is neither the father in law nor a female.