"Character is what you are in the dark." That's true. Character is what you are really like, not the coverup that you allow others to see. Character is what guides your actions and produces the words you speak. Character is your unique identity, the sum total of your individual characteristics.
Another perspective might say that perception is more significant as a determinant of identity, at least for an observer. Also, given time, and practice, the assumed persona can leak into what the individual considers his/her real personality.
There are character archetypes, most individuals would combine aspects of multiple archetypes - one is not constantly the Grouchy Old Man or the Innocent Blonde.
Incidentally the quote in the reference is by the character Lord John Whorfin - from The Adventures of Buckaroo Banzai - the whole quote goes
History is made at night. Character is what you are in the dark.
Buckaroo Banzai was an insanely creative film from 1984 where Buckaroo, a neurosurgeon, physicist and rockstar has interdimensional adventures The film in itself references Japanese animation films before they became uber-popular, and draws names & places from Thomas Pynchon's "The Crying Of Lot 49"
The book itself, referenced the 1960s significantly. In the words of John Hollander,
“a scathing...look behind the political events and historical figurations that led America into the mess that was the mid-sixties.”
Another memorable quote by Lord John Whorfin goes "If there's one thing I hate, it's to be mistaken for somebody else." That sums up character in itself.
Circling back to the classic cartoon - one would never like to be known as a dog on the Internet unless one chooses to be, or one is, in effect, a dog of sorts.