It's well-established by now that Rhue's sword, be it Shadow or Illuminati, has the power all such swords seem to have : it absorbs the auras of other wanderers. We might be wrong, that power might belong to Rhue himself, but that has little incidence on my point here.
While in most cases this happens when Rhue kills a wanderer himself (which was shown with Jed), it has also been proven to happen when Rhue is merely near someone who dies/loses their aura (Cade in the Jopaga bust).
If we take Rhue's memory at face value (which I'd rather not have to do, being convinced that his memories are muddled at best, and probably outright fake, him not being who he thinks he is... but what other choice do we have at this point?), then he was standing at the river bank when Serena got jumped by the beast and knocked into the river.
Now, had she died, either immediately, or sometime thereafter, is it inconceivable that Rhue, focused upon her as he was, could have in fact absorbed her aura from a distance?
I guess I just like the poetic value of Rhue searching for Serena all this time (although "Rhue" as we know him probably only started to exist around the same time E1 started) while she was in fact part of him, quite literally.
This might also have something to do with how he actually got his sword, the one he's so deeply attached to... Could the sword be some kind of materialized aura?
Just a few useless foreign pennies for your thoughts
