She was originally supposed to be a villain, but the writers liked her too much and ended up changing her role. I feel like this change was probably detrimental to the entire story and a lot of the characterizations. If she had been an unsympathetic character and an evil manipulative obstacle, the book 1 romance subplot between Korra and Mako wouldn't have made both lead characters look so awful and unbearable.
Of course, if they had actually used her as a villain, it would've been cliched and dull. Her character design just screamed femme fatale and her introduction got people suspicious. The fact she turned out to be such a kind-hearted and forgiving saint after all was pretty surprising. But unfortunately it also left her with barely any character, or flaws, or conflict. She was eye-candy but didn't have much else going for her.