What is the opposite word for alter ego?

There isn't a single perfect opposite for "alter ego." However, depending on the context, some good options include:

For a contrasting persona:

* True self

* Real self

* Authentic self

* Inner self

* Actual self

For a contrasting identity:

* Original identity

* Primary identity

* True identity

For a contrasting aspect of the person:

* The self (this is general, but can imply the absence of a separate persona)

* The individual (similar to "the self," but emphasizes singularity)

The best choice will depend on the specific context and what you want to emphasize.