What are Juliet's inner characteristics?

There is no description of her except that she is young, barely into her teens, and very pretty. Renaissance paintings of the time are the best indication what people looked like then.

Since she is a character in a play and therefore can be played by any number of different actresses (or even actors) she looks exactly like whatever the actress playing her looks like. If the actress playing her has long black hair, green eyes and pale fair skin, that's what she looks like. If the actress playing her is Rihanna, she doesn't look like that. She looks like Rihanna.