Hopefully you don't mind a non-Astartes joining your crew. I am aware that Ioana may sound OP (although I've tried my best not to make it so), which for a normal human she probably it. However, in my head she is not significantly better than a Space Marine. She's probably a better shot and more nimble, but equally a bolt round to the chest a Space Marine could probably survive whereas she would dodge it (well, that or die). I've also toned down her weapons as the Exitus rifle seems several tiers above a bolter. However, a sniper (like scouts have) that she can wield with the same dexterity as Space Marines can wield bolters seemed still in keeping but also about the same tier.
I think it's great! It sounds like an interesting character. I can see my renegade Raven Guard PC working pretty well with her, although I'm sure she'll be much more stealthy than a hulking, power armored Astartes.