alter ego

Short definition from Wiktionary

Noun

1. 1
   An individual's alternate personality or persona; another self.
2. 2
   A very close and intimate friend.
3. 3
   A corporation used by a person to conduct personal business in an attempt to shield himself or herself from personal liability, and which a court may penetrate by "piercing the corporate veil" to impose liability on the person when they commit fraud or injustice.
4. 4
   Alternative spelling of alter ego.