OK I couldn't think of a good title so I went with a pun :)
The Totland arc is full of characters with complex personalities and motivations, but chief among them may be Reiju. Is she good and will join the Straw Hats' side? Is she full of shades of gray? Is she just a bad person in general? Let's get down to business.
Obviously, unlike her younger brothers, Reiju isn't heartless, as she never physically attacked Sanji and ended up helping him escape due to feeling empathy for him. She also assisted the Straw Hats by saving Luffy's life.
Then again, she ain't really a paragon of virtue either. She's an assassin for hire. And with regards to Sanji, she did laugh at his abuse, if only because she wanted to avoid being targeted herself. That's not exactly considered to be a great trait in our culture, because to be a bystander to something is basically to greatly aid it.
So, is Reiju good or bad? Well, ultimately I think she is simply one of the most human characters in the series: wanting to do the right thing, but having shortcomings that mostly prevent her from doing so. Other than the high-tech assassin part, she's quite relatable. How that will factor into the story, I'm not sure. She could finally overcome her shortcomings and switch sides, she could both fight and help the Straw Hats like Mr. 2 did, or she could just be a villain. What are your thoughts?