Principal investor is a term of art in the industry. It has other financial meanings as well.
Principles are ideals you adhere to or uphold. They are also concepts used to implement a design. Principal and principle are homophones, and they are commonly mixed up.
https://www.law.cornell.edu/definitions/uscode.php?def_id=12...
https://www.investopedia.com/terms/p/principal.asp#understan...