The ability to extend your lifespan by stealing other people's destiny is rather unique... perhaps planescape has something that could be used here. I remember there was a sect there focused on extending one's life through dubious means.
Also, do you think a person killed by deathnote can be ressurected or will "maximum lifespan" clause come into effect?
Rouge, Possibly with aristocrat and/or Assassin. Not sure how to do the skinchanger thing (maybe variant chaining, If it would work), but wild Cohort would cover Nymeria.
Instead of Aristocrat, a Noble (PC class from dragonlance setting) could be used. But Arya didn't show any skills connected to those. Also, having Nymeria is a symptom of being an untrained skinchanger - at one point Arya managed to compensate for temporary blindness by borrowing sight from a nearby cat. Any way of doing it without having to take an entire class for it?