I probably phrased that badly. I meant that you could take her out of the episodes, and the overall story would work just as well, if not better. You'd have to find something else for Roy to do, but just make Thea as the person who gets injured (maybe she was going to meet Roy or something and becomes a casualty) and you give Roy something to do with a better character. BC doesn't need a sidekick. She's interesting because of her identity, Sin adds nothing to the character.
The name Sin makes no sense as a reference to a comic character that is almost nothing like the annoying TV version, with the only similarities being their gender and the color of their hair. Like I said, if Sin didn't annoy me and actually had a point, I could ignore that, but Sin being super annoying and pointless makes her name an issue.
As for why I find her annoying, what else can I say? She is a complete waste of screen time, her "role" is completely unneeded and its not helped by having a fairly mediocre actress playing her, not to mention the fact that for what she is she's not written well. Every time she was on screen, it felt like the TV show was wasting my time and I just want to see her viciously killed off as soon as possible. The show has enough interesting characters that the annoying punk/street urchin cliche is just wasting screen time. She's also nothing like Roy. Roy has a personality that isn't grating, and he has had character growth. He also has real motives and feels like a real character, not a cliche.