I assume that dual-blade dude could actually be a member from Tira's Birds of Passage group who are ... assassins... and whose organisation was destroyed during the Evil Seed event... which would fit the timeline perfectly. Maybe he's the head of the organisation who lost his mind because of the Evil Seed. That should not have happened yet in SCVI...
And as I posted somewhere else, judging from the trailer, he seems to wear similar gloves to Tira's armour gloves from SCIV and SCV, has his boots stuffed with feathers and all. The only thing not to coincide with my theory is that in the Wikis, I read that the ring blades were favoured (!) by the Birds of Passage and he obviously does not wield a ring blade. On the other hand, he might have priviledge to a different weapon for being their leader. Or they could simply retcon this. Also, imo his dual blade bears some resemblance to Tiras ring blade aesthetically... just a little, though...
What do you think?
EDIT: I re-read the Wiki and found out that only "Grim Bird"-ranked assassins would wield a ring blade, so that would work, too...