When she was a Ga-Matoran, Banshee was an airship pilot. After a long shift she was about to head home when, out of nowhere, a powerful strike flung her Pakari off her face. She was awake just long enough to see her mask crushed under the silver clawed heel of a large red Dark Hunter.
She was then taken to Odina. She awoke to find a Tryna replacing her Pakari. Before she could do anything she felt a surge of pain. For the next month the screams of the matoran could be heard across the island, becoming more animalistic by the day. The mask given to her by the Dark Hunters feed her a cocktail of liquid protodermis with small amounts of Hordika Venom and Pit Mutagen. The next hundred years of this slow mutation were done in silence, as her body got used to the toxins.
Even though she rarely makes any sound, if someone attempts to remove her mask she will wail just as loud as the first day she was "recruited" by the Dark Hunters. The other Dark Hunters gave her the name Banshee for that first sleepless month of her mutation.