I have a slight bias towards the NPCs that make me laugh over the NPCs that are just kind of... evil. There's nothing wrong with Vivi she's just taken a new role which is less entertaining, though she fills the role well enough.
Edit: Saya is officially a sort of anomaly, her inclusion primarily because, as somebody said, a legitimately helpful AND powerful NPC is a nice and interesting change.
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Accel R8
(This space for rent)!