The fact that he has things like Hyper Mode and even a freaking transition leap already makes him eligible to be playable. He'll basically fill the thief archetype(considering Yuri's the Ranger) like Lass does in Grand Chase, and judging by how "complete" the game will be in about 5 updates(OA Yuri, Misteltein, OA Seulbi, OA J and OA Misteltein), you just know they're going to need more stuff to add.
I believe they talked about having groups other than the Black Lambs, (See also: The "????" group beneath the Black Lambs in the PVP character select.) and according to 2nafish, Nata is a member of the Vultures' "Eraser" squadron. Basically criminal closers that are doing community service to get their sentence reduced or something of the sort.
I wish they'd lay off the sword users since we already have two playable and even another dual wielding closer NPC in Area 4. But whatever.....
Considering the story currently "ends" at G Tower, you think maybe the second story will involve the Erasers so that they can justify "unlocking" more characters for use? Like, maybe in the next story arc, some of the Erasers suddenly rebel against Union and the Black Lambs are tasked with capturing them, and then they realize they messed up or something so they'll decide to help out or something. Maybe have the rebellion happen in the middle of another dimensional invasion too.
Also, there's another dual-wielding Closer in G Tower? Goddammit, if all of the NPC Closers are dual-wielding edgelords, then I regret ever bringing up the possibility of them being playable.