a different version of oneself
alter-ego - second self
The term ‘alter-ego’ is used to describe a part of somebody’s personality that is different to what people usually see or think of you as being. In that respect, it is like a second self. That meaning has also been adapted to refer to a friend who is so close and trusted that you almost see them as a second version of you.
Claire is like my alter-ego in the office. I give the orders, and she makes sure that they are carried out.
When we were kids, Peter and I were like each other’s alter-egos. We did everything together.
David is usually a nice guy, but when he drinks it brings out his alter-ego and he becomes quite belligerent.