Formal:
* Nationalist: A strong advocate for their nation's interests.
* Loyalist: One who is devoted to their country and its government.
* Patriotic: Having or expressing love for or devotion to one's country.
* Countryman: A fellow citizen or inhabitant of a country.
Informal:
* True believer: Someone who strongly believes in the ideals of their country.
* Homebody: Someone who loves and feels a strong connection to their homeland.
* Defender of the flag: Someone who stands up for their country's rights and interests.
The best synonym depends on the specific context you're using it in.