The Banshee are a race of Shee more interested in the potential of the Grendel species than that of the Norn species popular with the rest of their kind. They originally had a great love for gadgets, which the other Shee thought to be un-natural. Perhaps the Lone Shee's exclusion from Shee society was due to an uncannily similar obsession with gadgets. Later, the Banshee became enamoured of the Grendels. They were the creators of the Banshee Grendels.
The Banshee diverged from the Shee due to experimentation with Grendel D-DNA, eventually left Albia, and fell into Shee lore only to discover the Shee Ark millennia later.
They are an anti-Shee race - if you like Star Wars a good comparison would be Banshee == Sith.