Necron is like...death or something. He wants to make everything nothing so he's like the Grim Reaper or something. "The Eternal Darkness" as his title is I believe.

And yea, in the end, it's hard to say who is talking. I like to believe it was Garnet (I found it never made sense as Mikoto since she went on, count it, one, airship ride with you and had no other journeys besides that with you and well it could be Vivi, but it seemed more intimate like what Garnet would say than what Vivi would say to me, but I guess it could just be a collective thought of everyone). And Vivi doesn't die. I believe if you watch the ending movie, Vivi has 6 children (as seen when they run by Puck). Then when it shows everyone in the seats clapping after Zidane reveals himself, there are 7 Vivis, so one must be the original Vivi and the other 6 his children.