From the Sonic Wikia: Keeping with the Egg Boss aesthetic, Nephthys appears serious and territorial towards her enemies with the intent of regional domination. In truth, she is a motherly, compassionate soul who believes Eggman to be unstoppable; to provoke him is to doom everyone else. She is protective towards her friends and innocents, willingly becoming an Egg Boss to keep Eggman from taking over by perpetuating an eternal stalemate with the Shamar Freedom Fighters. Nephthys is also modest and takes full responsibility for complications that arise, even when in the face of Eggman himself.

So, I'd imagine her having a mature sort of voice, like Jennifer Hale's Killer Frost from Batman: Assault on Arkham.