What was Captain secret identity?

It's impossible to give you a definitive answer to "What was Captain's secret identity?" without knowing which Captain you're referring to.

There are countless fictional Captains in books, movies, TV shows, and comics.

To help me figure out which Captain you mean, please tell me:

* What kind of media are you interested in? (e.g., comics, movies, TV shows, books)

* What is the name of the work featuring this Captain? (e.g., Captain America, Captain Marvel, Captain Underpants)

* What is the genre of the work? (e.g., superhero, science fiction, fantasy)

Once you provide more context, I can tell you the secret identity of the Captain you're looking for.