Identity is usually personality. It's the technique used to match a character's inner self, using language that identify his identity. Who that person is and the nature of what makes that person exist.

Marilyn, that's a great answer. I suspect, by the way, that the person who asked the question is a student trying to take a shortcut on their homework. ;-)

Peter
(The guy who runs Flashlight Worthy)

LOL

I was hoping it was from a serious writer who was stuck on describing his/her protagonist

Regardless, Peter, I liked Marilyn's answer a lot - it definitely defines (oh, those Ds!)a book character! Glad you left this answer in, Peter!