My intial thought was that was that this was going to be an identical twins story with Chang Ho being one twin and big mouse being the other. But now I find his friend suspicious.

Big mouse could be a group of people not just one person you never know but its still early we need more clues

First ep ending had CH talking to us - the audience. If he is telling us the story, he could be BM by being an unreliable narrator, lying or keeping out parts - manipulating our perception of him. Just a theory haha

Additional suspicious character  (They might be Big Mouse or they know the real Big Mouse)

- Jerry

- The prison guard they always focus

- PCH's Lawyer friend

I believe real Big Mouse is a twin brother of Chang Ho. That’s why no mention of his family background till now.

Those who attacked his twin brother were killed by him one by one. No other reason for big mouse to go to this extent.

PCH is getting confused because he looks exactly like real BM.

Jerry might be the right hand guy of real Big Mouse. And passed on real names to PCH saying they are from anonymous source.

Jerry can’t be real BM as he is too obvious to be guessed as BM after 4th episode.

 SweetJoane:

Exactly A was multiple people honestly at some point it was nearly everyone also it makes sense because even the inmates have conflicting views and on big mouses appearance .

You watched PLL.

Guys what if Big Mouse is a twin brother of Chang Ho??

There's no mention about Chang Ho family background... 

 SweetJoane:

Of course i literally became a detective  trying to figure who A was just for them to pull that ending lol 

Me too i watched PLL, now i forgot who is A lol