My suspicion is that the copycat vigilante is Ji-young's biological brother, perhaps even a twin.
Chapter 1 says his mother was beaten to death in front of her children, so I'm guessing the siblings were raised by different families or sent to separate orphanages.
So far, the copycat's morality isn't incompatible with Ji-young's, so they may even become allies. It will be even more interesting if they are identical twins.