Nosphaleen had more than enough Mature Aether, it was all stored within her body. But she was in an odd state.
She wasn't a beast even though Sylas had cheated somewhat to get his Profession to recognize her as one. For all intents and purposes, Nosphaleen was a humanoid. The path of beasts wasn't one that worked with her.
It was a bit more complicated than the way it was being described here, but the short of it was that Nosphaleen didn't have a Class, and she needed one in order to take a step into Level 10.
Sylas had neglected this matter because the Basilisk King hadn't needed a Class. But what he hadn't considered was that the Basilisk King's Lineage was its replacement for a Class.
Everything in this world acted on a set of laws known by experts as Gene Principles. Whether it was Skills, Professions, or even Classes, they all functioned through a mesh of Genes and its deeper, underlying layers.