what is the word that describes someone who keeps their real personality secret?

Incognito, via Italian from Latin incognitus, quite literally "unknown".

adjective
Without being known; in disguise; in an assumed character, or under an assumed title.

adverb
Without revealing one's identity.

noun

1. One unknown or in disguise, or under an assumed character or name.


2. The assumption of disguise or of a feigned character; the state of being in disguise or not recognized.