What we know about Assassin's Creed: Unity so far is that it looks amazing, it's set in France and we'll be playing as Arno Dorian, so this latest trailer gives us a look at some of the other characters. 

This time, the focus (or at last some of it) is on Elsie, a member of the Templars, who for some reason Arno is saving, despite the long-running war between their two sides. We presume all will be explained in due course, as Ubisoft state that Elsie is "determined to secure her place in the Templar dynasty [which]...leads her to cross paths with Arno and establish an unlikely bond with him". 

They also promise that she's central to the story but she won't be playable, at least not yet, but you never know what's in store with DLC these days.

We also get a look at the Paris of the French Revolution as stunningly rendered by the developers, and with so much detail, it would be hard not to see the city as a character in itself which will surely have a role to play.