It's kinda weird why a hotshot Mafia boss would call himself big mouse, seems underwhelming; a mouse usually feed on leftovers of humans they are basically the lowest why would someone call himself that. I think that big mouse isn't someone specific, maybe it's a fictional character that various people borrowed it to do bad deeds without implicating themselves throwing the blame on big mouse, maybe some people in the prison used big mouse persona to do their bad deeds, maybe even the mayor used big mouse to rob lala capital, since they are basically enemies. 

Updated the OP.

The preview for episode 5 is very interesting.

 SuspiciousAfternoon:

i can just feel it Chang Ho is big mouse, previews for next ep are usually not to be trusted fully but it points to him really being BM

i don't think he is, if you were writer, would you pick Chang Ho main lead character as big mouse?

it have to be someone we least expect is big mouse

For some weird reason I wonder if that one guard with the cap is big mouse? I think this will either be proved or disproved next episode cause I don’t know why changho would trust the guard and use the names given by him over those given by the mayor- unless the plot goes completely in a different direction he is either going to give the right names or the wrong ones, and if he’s got the right ones then who would he trust over the mayor? One possible solution is that changho has figured out who big mouse is cause then he would trust the list given by this person- he’s on the track of it most likely with his idea that it’s someone close in the prison. Whoever this person is seems to be on changho’s side and is benefiting from him being mistaken for big mouse and is helping him maintain this illusion. This is the reason for my suspecting the guard cause he’s in the prison and had access to the prisoners that have died plus all the inside information on all sides to help changho maintain this illusion, the way he’s been focussed on camera a bunch of times with his cap low and watching and is just generally in the background of most important events is so suspicious- he’s  been made to blend in whilst also being a continuous visible presence, this seems so cinematically purposeful.  The other alternatives are Jerry (he’s also a little suspicious) or Changho somehow (multiple personalities or the audience has been fooled by the narration and he’s been acting the whole time)

 Aklima:

it's not same cast, quite disappointed that they didn't carry on finished of Pretty Little Liars: The Perfectionists 

They should leave it alone, not making another Pretty Little Liars: Original Sin

Whaaaaat there a new season???!! And they changed the cast I’m definitely not watching it won’t be the same 

 SweetJoane:

Whaaaaat there a new season???!! And they changed the cast I’m definitely not watching it won’t be the same 

As you know Pretty Little Liars have seasons and they are done for good now (those who want to know who is A, tell me so i can share answer) . For Pretty Little Liars: The Perfectionists, its different cast but still have two main leads which is Mona and Allison in it however they left cliffhanger ending so its clearly they need season 2. Out of Nowhere they make Pretty Little Liars: Original Sin which is they uses very similar plot as PLL but whole completely different cast.

 Aklima:

LOL it is so funny that we supposed to talk about who is Big Mouse but how the hell we come to the so off topic about Pretty Little Liars   XD

LOL we've gone completely off topic

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.