I honestly have no idea the story has a lot of plot holes.. in the raws it shows the mean “twin” appearing out of thin air which shows it’s only in the nice “twins” mind. At some point the mean “twin” becomes the main personality and so the nice “twin” also appears out of thin air. I didn’t read all of it because it was just odd. If you want I can read it all and give a full spoiler of all the released chapters?

Okay so I read some of the raws and to summarize it all:
It’s basically a schizophrenic/DID situation. The nice “twin” and mean “twin” are the same person but the main/original personality is the nice “twin.” There’s a panel where the nice “twin” hugs the “mean” twin and a passerby sees the nice “twin” by himself, hugging the air.
For the physical changes it’s more than likely makeup and contacts! ^-^