General terms:
* Alter ego: This is the most common and generally accepted term for a second identity, often associated with fictional characters.
* Persona: This term emphasizes the outward personality or role someone adopts.
* Pseudonym: This refers to a false name, often used by writers or artists.
* Alias: Similar to pseudonym, this refers to a false name used to conceal one's true identity.
More specific terms:
* Doppelganger: This implies a near-perfect physical and/or personality resemblance to the original person.
* Shadow self: This term is more psychological, referring to the hidden, often negative aspects of one's personality.
* Avatar: This term is often used in online contexts to describe a digital representation of oneself.
The best word to use will depend on the specific situation and what aspects of the second identity you want to emphasize.