Okay, so, while I don't care about the whole Ninth Circle BS, I do think that Emiko is a really compelling villain. I'm excited to see how we end this season.
At first, Emiko tries to play it off like, Dante is forcing her to do these things because she owes him. However, later on, we find out that not only is she in the Ninth Circle by choice, but that she's running it.
Back when Oliver thought she was being used, she told them that Dante was looking for Uranium, so the gang stakes out all the places where he could steal it, but came up empty. Because, Emiko lied and he was actually looking for Sarin gas. They've got a whole plan to inject a bunch of drones with it and then release it.
Thankfully, they figure out the plan and are able to get to the drone place just before they are launched. However, they are cloaked somehow which makes them invisible, so Oliver can't do anything until Felicity can make them appear. Eventually, she gets all but one. In another bit of good luck, the one she couldn't get in time was released into an empty building. So, basically, this was just a trial run it would seem.
While Emiko is doing all this, she somehow also has time to try to take down Laurel. She finds photos of her from when she was the Black Canary and working with Diaz and leaks them to the press. Not a great look for the DA who is already on thin ice.
Plus, Felicity hires Elena back to the CTO of whatever company she's got going on now. They are working on perfecting the Archer program. If you've got DNA from someone you can track them instantly in real time. Unfortunately, back when Emiko was in the bunker she got a look at it and has now decided that the Ninth Circle needs to steal it.
Also, we get a bunch of Emiko flashbacks and we see that Dante has been training he since she was a child. We also find out that she went to her dad when she was an adult to ask some business stuff, but he denied her. Shortly after that, we find out that she knew the Gambit was going to crash beforehand.