As a twin I would like to say People don't know that twins can start to look slightly different over time, especially if they like different lifestyle's or diet's
There were a two astronauts, identical twins, one was on the space and the other here on earth. Then when it was time to go back on earth, the other identical twin becomes slightly different. Vergil was on Limbo or Hell before, then mundus controls him. The same is this on Vergil, stress can really change the look of a person and vergil was once a side character but thwn Dmc 3 came out and he became a fan favorite character.