Isn't it blatantly obvious?
Kitty Riley is Moriarty. Richard Brook is merely her sociopathic actor friend who acts as her version of Watson. Initially Sherlock believed Moriarty was a woman, it is shown from Kitty's apartment that she is a world-class painter (two of Sherlock's cases involved fake paintings), she has 'Make Believe' on her wall - how foreshadowing is that! And if you go through the apartment scene it all makes sense! Look at her handwriting on the article which she is editing, look at Moriarty's hand-written notes and then think about the classic Moriarty from the books: always keeping to the shadows, orchestrating in the darkness!
Kitty Riley must be either Moriarty or a prime accomplice.

"There are two types of fans. Type A, catch me before I kill again."
"What's Type B?"
"My place is just a taxi ride away"
"Guess which one I am"

Creepy.